I find the screen tearing (eurgh, I hate it, especially in racing games as it's so distracting), framerate drops, ugly low-res self-shadowing and poor texture filtering (leading to shimmering) to be slightly offputting in Gran Turismo 5 plus about 80% of the cars are weakly textured and use a lower polygon count then the premium models. Factor in the lack of a proper dashboard (cockpit) view and Forza 3 and now 4 are much more graphically consistent and pleasing to me overall. Where GT scores points over Forza, is it's clever use of camera angles during the replays which make the game look a lot more impressive than it really is.
I think Polyphony Digital made the wrong decision aiming for 1080p this generation as the PS3 clearly struggles with it. It's a great technical achievement, I'll admit, but the game has plenty of rough edges that mar the overall look because of it.
It's only the photo modes in both Gran Turismo and Forza that the games make my jaw-drop as they obviously deliver a superior image quality that neither games can deliver in real-time.