Both types of valves end up performing the same thing - preventing you from being scalded if the incoming water volume/temp changes; they just do it differently. One adjusts between hot and cold balance by use of a thermostatic control, the other, you move a lever to the place you want, and if the pressure changes the flow, it shuts down part of the flow to keep the balance the same (and thus, in theory) the temperature. The temperature controlled valve will adjust as you start to run out of hot water, or from season to season as the cold water changes (as in winter to summer). A quality valve can be set to your ideal temperature and left there winter to summer; a pressure balance one can't. Where I live, the winter incoming cold water approaches freezing, while summer can be in the 60's.