Honestly I found GT5 had so much done to the cars just to make them harder to drive it was stupid.
Like having the default brake balance at 50-50, so every time you buy a car you have to go in and adjust it so the damn thing doesn't oversteer going into EVERY corner. That and the suspension settings are totally buggered on a lot of the cars too, so I had to spend time getting them right.
It took me two hours to get my AW11 MR2 to handle even remotely like the real thing. When it should have right from the get-go, especially considering it was all stock.
I found Forza 3 handled better from square one, and actually seemed somewhat like driving a real car, closer than GT5. Though neither of them handle oversteer correctly, it always feels wrong.