PGR always has a more arcade feel to it, but much of the physics is still grounded in realism. I personally am in the camp that finds the PGR series of games more fun that your average sim, power-sliding a Ferrari 575M at 120 MPH just has some strange appeal to it.
Forza 1 is still a blast though, and I still play it occasionally. Thing is, PGR is just a lot more fun.
Wanna rummage through the bargain bin for another interesting racing title? Give Midtown Madness a look... I thought that game was fun for a while, but its really easy to beat...