To me the pre-baked lighting, 2d wheels, shimmering and complete lack of any damage model ruined GT4 for me. Forza was just so cool to be running the track at night, have the dynamic shadows play over your car, reflections in realtime, and watching your calipres glow through your fully 3d wheels.
To me GT4 looks best at first glance, while Forza looks better the more you play it and the more you notice. The colour scheme is down to opinion, I certainly have no problem with the bright vibrant colours in Forza they are very easy on the eyes, GT4's look really good too, though a little drab and the poly's in the backgrounds are alot lower.
I think powderkegs pictures speak louder than words.