Forza's graphics are much better, but honestly the physics and overall driving is betterin Gran turismo 6…if you play with a proper wheel setup. Don't get me wrong, I think forza is far more fun as a racing game as races just feel more exciting, but if you are looking for s tool to actually help with driving (car control/etc) I'd stay away from forza as its not very realistic. Gran turismo's physics and tire model are better. I think the next gen GT on the ps4 is going to blow forza away.
But these days I play neither as I'm hooked on the best simulator on the market. Assetto Corsa which is actually coming out on both consoles next year.
Here's my current setup: