The sort of where, what, and why of pressure-balanced versus thermostatic:
Pressure-balanced or thermostatic temperature control valves are code-required in bathroom plumbing because they eliminate potential scalding and cold water shocks that can occur in a shower.

If you are using the shower and a toilet is flushed, as the toilet uses cold water to refill the tank, the pressure in the cold water line drops a bit below what it was when just the shower was running. If you had a non-balancing valve, you’d still get the same amount of hot water that you originally were getting, but with the drop in pressure in the cold water line you’d have less cold water coming out of your shower head, creating a potential for scalding. Vice-versa, if someone turns on a hot-water faucet elsewhere in the house, the hot water pressure drops and you get a shower of mostly cold water.

A pressure-balanced shower valve is designed to compensate for changes in water pressure. It has a mechanism inside that moves with a change in water pressure to immediately balance the pressure of the hot- and cold-water inputs. These valves keep water temperature within a couple degrees of the initial setting. They do it by reducing water flow through either the hot or cold supply as needed. Because pressure balanced valves control the temp by reducing the flow of water through the valve, if your plumbing supply is already struggling to keep up with the three shower heads and nine body sprays that you have running in your shower, if a pressure balancing valve kicks in and chokes down the water supply to keep you from getting scalded you could end up with insufficient water flow out of the heads in a multiple shower head setup. When it comes to volume control, in terms of being able to turn on the water a little or a lot, for the most part pressure-balanced valves are full-on when water is flowing or full-off when the valve is closed. Flow-wise, think of them as having no middle ground.

Where flow and volume control are important, as in a shower that requires a high volume of water, a thermostatic valve may be the better choice. They also control the temperature, but they do not reduce the amount of water flowing through the valve in doing so. Thermostatic valves are also common with 3/4" inlets and outlets, so they can pass more water through the valve than a 1/2" pressure balancing valve.

Which should you choose?
In a larger multi-outlet master shower, while a 1/2" thermostatic valve may suffice, a 3/4" thermostatic valve might be the better choice. But it does depend on the design of your shower and the volume of water that can be passed through your houses supply lines. In a secondary bathroom, or in a basic master where you have only one head, or the common shower head/tub spout diverter valve, a 1/2" pressure balancing valve would be fine.

If you want individual control and wanted multiple valves controlling multiple heads, then you could use multiple 1/2" valves instead of one 3/4" valve and all would be just fine.

What do the controls on the valve actually control?
While it may vary, a pressure balanced valve is normally an "all in one" valve with only one thing you can adjust…the temperature. The valve usually just has one rotating control (lever or knob) where you turn the water on, and by rotating it you set the water to a certain temperature. Each time you turn the valve on you’ll have to set it to the same spot to set it to your desired temperature. For the most part you really don’t control the volume, just the temperature. With the valve spun a little bit, you'll get 100% flow but it will be all cold water. With the valve spun all the way, you’ll get 100% flow, but it will be all hot water. Somewhere int eh middle you’ll find that Goldilocks "just right" temperature, and it’ll be at…you guessed it…100% flow. So with a pressure balancing valve, you control the temp, but when the valve is open, it’s open.

A thermostatic valve can be all inclusive in terms of control (volume and temp) or just be temperature controlling. If it’s just temperature controlling, you will need a separate control for volume or flow. Example, with an all inclusive you’ll have two "controllers" (knobs or levers) on the valve, one to set the temperature and a separate one to set the volume. In this case you can set the temp as you like it, then use the volume control lever to have just a trickle of Goldilocks water come out of the valve, or you can open it up and have full flow of Goldilocks water coming out of the valve. You can leave the temp where you like it when you turn the volume off after you’re done showering. The next time you shower, turn the volume on, the temperature is already set. Some thermostatic valves are just temperature valves with no volume control. You’ll need another valve/control to set the volume. Read the product description carefully to see what you're getting.

What size valve should I get?
Yes, valves actually come in different sizes. The size refers to the size of the inlet/outlet nipples on the valve. For a basic shower, a 1/2" valve will suffice. For a larger multi-head arrangement, a 3/4" valve would be better. Realize that you’ll need a water heater that can supply the volume of heated water you want coming out of the heads, so don’t forget that when you build or remodel. Also realize that if you’re remodeling and have 1/2" copper running to your shower, capping 1/2" copper supply tubing with a 3/4" valve provide you with much benefit as the 1/2" tubing is the limiting factor. You can, however, cap 3/4" supply tubing with a 1/2" valve or a 3/4" valve.

Is one better than another?
Thermostatic valves are "better" in that with them you can control both volume of flow and temperature, so you have more control, and they hold the temperature to a closer standard (+/- 1 degree). They also perform better if you are running multiple outlets in the shower, as they do not choke down the amount of water in order to control the temperature. But you pay for that added flow and added control. Pressure balancing valves can be had for about $100-$200, thermostatic valves can be twice that amount. And more.

Will I suffer with a pressure-balancing valve?
For what it’s worth, when I built my house over 10 years ago I put pressure-balancing valves in my own house. While I have two outlets in my shower (sliding bar mounted hand-held on the wall and an overhead 12" rain shower head on the ceiling), I have a two separate pressure-balancing valves, one valve for each head. With both heads going in the shower, I notice no loss of flow in the shower when the toilet is flushed and the sink faucet is turned on simultaneously. I also notice no change in temperature. So they work for me.

If you are remodeling, if you have your existing sink running and you flush the toilet and notice a drop in volume coming out of the sink, then a thermostatic valve might be the better choice even if you're not having a multi-head setup installed.

If, as part of the remodel, you plan on running new supply lines through your house to the new bath, then properly sized runs will take care of that flow restriction and you can probably do a pressure balancing valve instead of a thermostatic.

So in a house with tricky plumbing, or with a restricted water supply, or with multiple outlets running off of one supply valve, a thermostatic valve might be the safer choice.

Mongo

First, I'm obviously biased by my experiences both as a user and an installer.

For a small shower, a single shower head on an arm high on the wall will do you quite nicely. In a tub surround the shower head up high with the tub spout with diverter down low is still king. Nothing wrong with it at all.

A question I often get is about handhelds. Now when I was a kid handhelds were trash. They leaked, they had the 4-in-1 massage heads where the only thing they massaged was your eyeball because there always seemed to be a pinprick stream of water that would leak out the fitting and nail you in the eye. After a few weeks of use they'd be spraying water out the side seams of the head, up and over the shower curtain, soaking your bathroom floor. Ugh.

Not any more.

Handhelds today are pretty darn bueno. The heads are the same quality as those that get mounted up high on static arms. The hose fittings no longer leak. The hoses no longer twist upon themselves like demented strands of spaghetti.

The big benefit of handhelds is the length of the hose allows you to wash/rinse any part of your body without having to be a contortionist. For shaving legs you can take the head off the bracket and hold it in your hand, or mount it on another wall bracket at knee-height. Or you can use the handheld and it's long hose to rinse off remote corners of the shower's walls when cleaning. Or when bathing kids. Or dogs. Versatility.

Handhelds are an obvious advantage in larger showers, but they can be of use in smaller showers too.

Slidebars versus brackets: In a master bath, it's possible the slide feature will be seldom used. You can still have one...I have one in my shower, my wife lowers it when she's looking for a dry hair shower. Other than that it pretty much stays up at my height. And we're 6'4" versus 5'1".

If you have kids, it's an advantage. Low when they are younger. Raise it up as they grow taller.

Different height users who are particular...I know a couple that resets the head height each time they shower. Although they have a small single-head shower, he demands it at his height, she at hers.

Slidebars can add visual clutter to a wall. But while they are functional clutter, in a small shower someone might prefer the cleaner look of wall brackets. A bracket up high. A bracket down low. Whatever you need.

While most companies' slide bars might be plastic or thin-walled metal, there are some that make them sturdy enough to function and be rated and approved as structural grab bars.

Rainhead? A true rainhead delivers a very gentle flow of water. Personally, if you're looking for a true rainhead, I'd recommend a minimum 10" diameter head. 12" is better. Rainheads generally have to be mounted parallel to the floor, as the water pretty much just "falls" out of the head instead of being sprayed by pressure. Were you to tilt a true rainhead, the water could just run along the tilted face of the head and flow off the low edge in a fat stream.

Due to the gentle flow, rainheads are a nice experience. Quite a bit different from the pin-prickish stronger flow of a traditional head. With the gentler flow, those with long/thick hair might find themselves running out of hot water before they are able to rinse shampoo out of their massive manes.

So I consider rainheads to be a nice secondary head, and I prefer them to be plumbed or mounted close to the center of the shower ceiling where it's easy to stand right under them, versus mounted on a wall arm with the rainhead close to the wall.

Rainheads have been modified, now there are ones with "turbo" functions, or air-entrainment, etc. Sort of halfway between a traditional standard head and a traditional rain head. You'll have to sort through that yourself as there are too many options.

Personally, I think a master shower will do just fine with a "standard head" handheld (can be a 4-in-1 head or whatever) on a long hose and a separate rainhead. That'll give you a functional shower plus the option for a soothing rain shower.

If a couple will be typically be showering together, then consider two one supply valve feeding a handheld head, plus another supply valve with diverter plumbed to feed either a second standard head or an overhead rain head. That will allow one person to shower at the handheld with one water temp setting, and another person to "standard" shower or "rain" shower (via the diverter) with a separate water temp setting.

Body Sprays: Personally, I consider them superfluous. I've used them...I think them a novelty. But there are folks who just adore them, so decide for yourself. Do realize that body sprays can pop the plumbing cost through the roof because:

Showers are required by code to have a minimum 2" drain line. Now you can have two 2" drains, or a single 3" drain, but typical is a single 2" drain line. "2 inch" and "3-inch" defines not just the size of the drain opening, but the diameter of the drain branch under the floor.

Code assigns values to drain lines for how much water they can carry away from the shower. A 2" drain line can evacuate 6DFU (drainage fixture units), a 3" line 20DFUs.

Miraculously, shower heads are assigned values as well. Each shower head is assigned a value of 2. A shower head is a handheld, or a fixed head, or an INDIVIDUAL body spray head.

So with a typical 2" drain (6DFUs), you can have three heads in a shower (3 heads x 2DFUs per head = 6DFUs). Since body sprays are usually installed in multiple groupings, installing body sprays can really ramp up your plumbing requirements, both for water supply lines, the water heater, as well as the drainage lines.

I know some inspectors that count the heads and multiply by two and there you go: a rain head, a handheld, three body sprays, that's 5 times 2 = 10DFUs, you'd need ether two 2" drains or a single 3" drain.

I know other inspectors that look at the supply valves and/or diverters and would recognize that the shower is plumbed so that only the two shower heads OR the three body sprays can be on at any one time...two heads times 2DFUs per head = 4DFUs, OR 3 body sprays times 2 = 6DFUs, a single 2" drain will suffice.

All-in-one shower towers? Another animal that I really can't discuss since they can vary from A to Z.

Anyhow, I'm out of coffee, so it's time to go.

dealyard.com - if you can find what you want, the prices are just plain unbelievable on high end products, in a few cases, I paid 10% of list for discontinued styles of high end European items. we ordered seven faucets, one shower set, one shower/bath set, one roman bath set from here all arrived in perfect condition. was in three separate boxes, fed ex, the company only had signature required on one of the three boxes, so two boxes were left outside our home one day, and the third box needed signature - all three should have needed signature - weird glitch. Would suggest calling the company after placing order to make sure they require signature - I didn't do this.

faucetdirect.com - ordered jacuzzi air tub and kohler cast iron bath tub. prices were the best for these items. they were shipped separately. it was snowing the day the kohler arrived and the ups driver said that dropping the tub to the curb was not included in the delivery, so the contractor plus plumber actually lifted it themselves. customer service said it was not their fault, UPS said it was not their fault, either way, I did pay for curbside delivery and did not get it, I would suggest when ordering large items like this to talk to the company ahead of time, which I did not do.

homeproductstogo.com - ordered three toto toilets. no problems. really nice local delivery company was used who insisted on helping us take the items into our home - I did not pay for white glove service, and got it anyway

overstock.com - kitchen sinks. no problems.

Hope this helps.