This is an old thread to dig up, but the title and discussion fit exactly to my remodel plans and questions. Here is the decision I reached regarding choosing Thermostatic vs Pressure Balanced:
Both types of valves are made with a temperature "memory" feature if there is a seperate on/off control knob. You just leave the temperature knob in the same position. But typical usage is to adjust the temperature as desired for each time in the shower. My wife likes to get some steam sauna effects going so she turns it really hot. I like it very warm if I've been outdoors on a chilly day, or need to relax some muscle aches. I like it cooler if I just finished a sweaty workout, or if the wife is out of town and the "urge" strikes.
My water heater is a 50 gallon size and holds a good reserve of hot water to last thru a shower. I barely notice any temperature change as the heater tank is continuously topped off with cold water, even if the shower in the 2nd bath is also being used. But if the dishwasher or laundry or lawn sprinklers kick on, I do notice a very sudden and uncomfortable change in temperature. The manufacturers of thermostatic valves never say how long it takes to readjust for the temperature change due to pressure steps, but I suspect it is long enough to be noticed. This is because the temperature sensing element has thermal mass which must be changed, then the expansion/contraction (tiny force) of that sensor must be "geared" or leveraged to drive the valve that adjusts hot/cold water flow (big force). This tiny force can overcome the big force if it has time to work on it. (An example is the lawn sprinkler valve. The solenoid by itself is not powerful enough to plug the raw water pressure force. But if the solenoid is used to open a small valve which then fills the chamber to push on the diaphragm that then pushes on the plug to stop the raw water pressure, it does work, but takes several seconds to accomplish.) A pressure balance valve uses the incoming hot and cold raw water pressure to move the mechanism that adjusts flow. There is no time delay for thermal mass to react, and it does not need the time to make leverage work as the raw water pressure has lots of force to quickly move the hot/cold flow valve.
Water pressure in the house is good; on city supply, and there is an adjustable pressure regulator where it enters the house. So there is enough pressure to operate the pressure balance mechanism. A well pump might not build up enough pressure.
I don't know if having 2 shower heads (one rain, one handheld) going at the same time will disrupt the pressure balance valve mechanism because of too low pressure on the pb valve's outlet side.
The extra $100 outlay for a thermostatic valve is not justified, in my opinion and circumstances.