The ID of CPVC is smaller than copper or galv, so size your system accordingly. You should change out your shower valve to a pressure balance. Showers have a max GPM (measured at 80psi). A larger line will not increase this. However, I would run 3/4" lines to the bath--I never run more than 2 fixtures on 1/2" (& remember CPVC ID is smaller).
The choice of water line material.....? Kind of a personal preference/water quality issue.