To me, the lighting alone makes it look better than Forza.Sorry.. but from what I've seen. The only thing that looks better than Forza is the photomode... IMO
Forza (while vastly improved from the Forza 3 days) still has that "a little too vibrant" "something's not quite right""that material should not be reflecting light like that" look.
But I'm sure there are people that prefer that look/lighting over GT's as well. Forza still looks great imo.