I'm trying to decide between thermostatic and pressure balaced
temperature controls for my shower faucet.

I hunted around through previous postings and got the impression that
thermostatic valves have a slower response than pressure balancing
valves. I was told by one salesperson that the response was "1
second" which seems kind of slow to me (though he said it like it was
fast). Another question I had was that if the thermostatic valve is
supposed to produce a fixed temperature output, what does it do when
the target temperature is impossible to obtain with the input water
supply? For example, if I demand 110 degree water but my hot water
pipe is full of 65 degree water, what happens? I was told by another
salesperson that the thermostatic valve merely sets a maximum possible
temperature. But if that's true, then when someone turns on the hot
nearby, or if the washing machine gets turned on, then it seems like
I'd get hit by a sudden blast of cold water. Is this true?

I got the impression that pressure balance acts faster than
thermostatic. And as long as the temperatures of hot and cold remain
the same, it should give the same output temperature no matter what
happens around the house. Is this better than the thermostatic? Can
anybody point me to information on how the pressure balance valves
actually work?