Couple of reviews from sim orientated site I found (in addition to above link):
http://www.virtualr.net/gran-turismo-5-review/
http://www.virtualr.net/forza-motorsport-3-review/
Both titles have pretty good physics but from a sim perspective (i.e. less hand holding and more accurate representation of car interaction with the road) GT5 seems to have the edge so far as I can tell from what I read plus my own play through.
It sometimes feels like Forza 3 could have matching physics but the balance leans towards gameplay vs true sim and it mostly feels like the game holds back from what it could simulate in the name of making races more fun. The road surface and interaction with it also seems superior in GT5. The surface feels awfully even in Forza 3 whereas in GT5 you can really feel the road undulate and the impact on the car's physics.
Again, both are good, but GT5, particularly as you advance, seems to provide the more accurate driving physics and road surface.
Neither - of course - matches the physics of the PC based driving sims. But GT5 comes a bit closer than Forza 3.
Try to be reasonable... its easier than you think...
