You lose pressure if the fixture is capable of using more water than is supplied, sort of like a finger over the end of a hose. With one showerhead, 1/2" pex would work. If you're trying to fill the tub as well, either don't use pex, or go larger. 1/2" pex is about the same ID as 3/8" copper. 3/4" pex is closer the 1/2" copper. With a 3/4" valve (expensive), I don't know why you'd want to throttle it down with 1/2" pex.