Gek54Yes I have played both. Not the final build of Forza mind you but from what I have played so far, Forza's physics seem more accurate than the final Japanese GT4 physics engine. The sound seems less muffled in Forza as well.
I am for one VERY excited about Enthusia. It is sounding like it will be the real GT4 killer as far as pure driving goes.
from what I've seen Forza has a poor portrayal of real life gravity, the cars fload a LOT when coming off of a jump, or run into walls/other cars. In GT4 I have only seen this happen at high speeds, in Forza, in the videos, I see it happen at lower speeds and on small impact collisions.