Yea Forza is more realistic for sure, you'd never be able to pull the stuff off in PGR 1-4 in real life. The only problem is with Forza is no car interiors, so basically a graphics thing. But if you really want to feel what its like to drive a euro/japan car, i do find american cars are without parts that make them handle much better. Again Forza is pretty sim, so your fun factor is pretty low, its more in tune with doing model cars, except now you can drive them and they're like their real life counterparts to the best of todays technology.
But again Forza 2 gathers dust, while i'm trying to play through PGR 3 which i've been avoiding since i got my system and that game with it. And its just mind blowing arcade fun, after awhile it gets old, and sometimes its nice to switch over. I'm still waiting for my replacement xbox to get its licenses updated before I can get some of my Forza 2 stuff i bought back in the game. So thats another reason i'm not playing Forza 2 over PGR right now.