In my opinion, Live for Speed (LFS) is far and away superior in terms of physics simulation than anything else out there right now. rFactor is nice due to being mod friendly, but the underlying physics engine stinks in comparison to LFS.
Everything about rFactor looks and feels "plasticky" after playing LFS. Hard to describe. I would suggest downloading the demo from rFactor and LFS and giving them a whirl. Tracks and cars, with the exception of four of the cars, are fictional in LFS. That said, I'm glad for this, as the tracks are amazingly well designed and quite diverse. Same goes for the cars. The Formula BMW in is sublime and provides much better races than the full out F1 car.
Are you planning on getting a nice wheel as well? A good sim is worthless without a decent set of wheels and pedals...