The internal size of 1/2" pex and 1/2" copper are different - the copper has a larger opening. But, with copper, you'll have elbows and other changes in direction. The pex is smoother. The volume actually delivered to the fixture may be close. Pressure should be the same. A larger pipe means after the hot cools off from the last use, you have more to purge before it arrives at the fixture unless you use a recirculation system (pump, usually). So, it is a tradeoff. If you have a large capactity tub or multi-head shower, you might want the larger pipe. I probably missed some points...wait for some other thoughts.