GT definitely has the better lighting. Forza 3 cars while being beautiful still have a plastic look...
On the physics side, I'm not sure but I know many real pilots use GT for training and not Forza. They even tried to put a GT champion on a real track and he finished third so it must be the most realistic...
Heck, you can't even connect a decent wheel like the logitech G25 on 360, so I don't even know how Forza players could say it's realistic (or do they play with the pad ?)















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