Back. The whole family's tall so when we remodeled the bathroom I had them raise the showerhead. Now, not only does it hit you in the face, but when rinsing your hair, it splashes over the top of the shower doors and gets everything about a foot out wet.
Yes, I realize that you wash all sides. The question is which direction is your default stance? I'm a back to the shower sort of person. I can retro-rationalize it as my back needs more cleaning as it's harder to scrub manually.