nKp is a much more difficult sim than LFS, though, so be warned. It's also mainly aimed towards openwheeler drivers at the moment, with only 1 tintop being included in the full game. The demo only lets you drive the F1600. That being said, it is a very rewarding sim if you're willing to put the time in to become good at it, and visually it looks a lot better than LFS.
As for rFactor, with the right mods and tweaks I find it feels a lot better than LFS and it's a hell of a lot more fun, but you really need to know what to look for and what to edit in the different files. The default content is shite and absolutely not worth bothering with. Also, online racing tends to be a nightmare with rFactor. With the countless mods and tracks, and then even different
versions of those mods and tracks floating around, you'll be hardpressed to find a populated server to race on. You really have to look at racing communities and organized racing if you want to race your favourite mods online.
As for iRacing, you can get a 1 month trial for $5 with the Radical promo. At least then you'll know for yourself if it's good or not, instead of just going by what you see and hear from other people.
EDIT: beaten by The Moose. Nothing new there...