There's not that many console "sims". Off the top of my head, GT5 Prologue, GT4 on PS2, Enthusia on PS2, Forza and Forza 2 on Xbox & 360, Race Pro on 360, Ferrari Challenge and SuperCar Challenge on PS3, F1 '06 on PS3, that's all I can really think about it.
I have all of those games except SuperCar Challenge (not available in North America afaik) and I don't have Forza 3 yet either, so can't quite comment on Forza 3.
As to what it most compares to, I would say there's no other game like it (referring to Forza 2 anyway). I've played through GT4 and enjoyed it greatly, same goes for Forza and Forza 2. With Forza 2, I spent majority of my time racing online with organized league racing and tuning various cars. It was very enjoyable and the car selection it offers with Ferraris and Lambos and such was enjoyable for me, same goes for the tuning aspect how they have engine swaps and such. GT5 may have this too, but by the time GT5 comes out, I'll get a lot of time out of Forza 3.