If your primary concern is mods then go for rFactor. Though I havent' seen many mods that are much different than the retail game. Another type of formula car (F3) doesn't really add much IMO.

Which do you prefer and why? LFS I think has the handling and physics down to almost perfection. The only thing I can think of that comes close is GT4 (No, I did not care for how ForzaII felt for some ...