Check the manufacturer's specs on the gpm of the things you want to run simultaneously. 3/4" pex might be too small. 1/2" pex will run one showerhead fine, but might struggle with two. You might need a 3/4" valve, then you might want to run 1" pex to it. It all comes down to the gpm demand with everything on. Without knowing that, it's all a guess which includes which valve you need.