GT5P is just basically a demo so its hard to have a fair comparison. Forza 2 beats GT5P by a longshot in almost every way. For a fair compairison you really should wait until the full version comes out but by that time Forza 3 will be out.
The physics still dont seem right in GTP5 at all. Everything always feels more realistic to me in Forza 2 compared to GTP5. You can even tune the physics in Forza 2 just right so you can pull a wheelie both tires or 1 tire. Also when simply taking jumps it does not feel real in GTP5 while it feels very real in Forza 2. The inside view of GTP5 is very nice and its one of the main features that Forza 2 does not have. GTP5 also has 16 players online while Forza 2 has 8. But you cant play GTP5 with 16 people until you beat the C,B,A events or use a game save.
Forza 2 is a full game so there is the ability to completely customize cars with body kits, black marks that stay on the road, many real life parts, swap engines, full car damage, swap drive trains making FWD cars become RWD, change wheel/tire width, wheel/tire diameter, incredible physics, intelligent AI, pick any color I want for my car, window tint color, paint my wheels any color I want, paint my calipers, 47 tracks, 12 different environments, leaderboards that automatically update after each race, sell cars online with other players, buy cars online with other players, turn collisions off or on, turn damage off or on, and much more. It also shows where your tires are wearing out instead of just showing green, yellow or red when they wear out. If you have too much camber you can tell by the tire wear. When you play online in Forza 2 it feels very rewarding since your car is extremly customized to what you like. Not just picking a car that only has stock colors and nothing else. Forza 2 is the best console sim I have played with WAY more options than any other game out there when it comes to customization.