The 3/4" will be more expensive, and if your supply pipes are not at least that size, it is probably a waste of money. A 3/4" valve is great if you have multiple showerheads/body sprays, or a really big tub that you want to fill fast. In either case, you can convert from one size to the other to handle whatever valve/supply you have. For a single showerhead, or eve a double, 1/2" should flow enough to satisfy you if the pressure is okay. If you have galvanised supply pipes in stead of copper, then at this age, you are probably looking at restricted flow. My unprofessional opinion.