My .02...
Forza...wow. Got it for $29.99 at SamGoody's and it is just...amazing. The feel of the cars is much more realistic in my opinion compared to GT4. In GT4, the car is stiff and seems like it never moves. In Forza, you can "Feel" the body roll and the movement of your car as it takes a turn. I also notably liked the braking. If you go into side view in a car like the BMW M3 race car or any other race car or car capable of 180+ and you go top speed or close to it and slam the brakes, you can see the rotors turn glowing red and how it effects your braking. The graphical detail is equal to GT4 and better in some cases. The interactivity with the cars is somewhat better than GT4 (just no photo mode). Being able to paint, decal, vinyl, and upgrade your cars is very fun for personalization. The selection of cars also makes the game fun. The sound and audio is MUCH better than GT4. Every car sounds unique and grat. the 'Ring on Forza is alot better than GT4's I think for the following reasons:
Much more scenic detail (notice any new BUILDINGS in Forza where the GT4 start and finish line was?).
The road/grass detail is much better.
I could go on and on, but I have to get back to playing Forza.