rFactor isn't the perfect game, but what is? The tyre model is severely screwy (like most ISI-based sims), but people like Niels do their best to fix this problem in the way they develop their mods. Aside from that, it's incredibly moddable, which provides a great diversity of racing, but pick-up races are almost impossible (because of the great diversity).
If you had to choose between rF and LFS, get LFS. But if you're looking for an addition, and you enjoy diversity of options, get rF
If you had to choose between rF and LFS, get LFS. But if you're looking for an addition, and you enjoy diversity of options, get rF