I have both LFS and Rfactor and i like them both.
However are a bit different.
And im not speaking about physics or mod quality (watch out for 2cv, ETCC 2003 and BMW M3 E46 mod, will be amazing...
)
I'm speaking about the "mindset" that you have when you play.
Rfactor imho explain all its fun when played in private servers with people that you know and that are good are driving. This is because the general level of players on public server is lower then lfs (due to not having things like team inferno setups imho), so there are noobs, wreckers etc...
However when played on championship is extremely fun, maybe for the more realistical aspect of motor racing, because it simulate real series on real tracks ( F1 1979 at nordscheife it's simply amazing!!).
On the opposite, if you want a fast run, lfs beat them all.
Because generally people on server are fast, or at least correct (or experienced a bit for not causing a crash at each turn), and the race starting process is faster: 5 laps, then again.
On rf usually servers are on pratice 1, then 2, 3 then qualy then warm up and then race lol
Rf is heavier about fps, and you need a good pc for run it well.
However graphic detail is higher then lfs (lfs seems a bit more natural on colors imho)
Netcode is good on both, no lag at all.
Both are good games. If you are a racing simulation player, imho you have to buy them both. There isn't nothing better on the simulation market.