Not raced the LMPS but have played both games a fair bit. There are 3 areas I can see where iRacing is better.
1.Multiplayer - its no.1 selling point over all other sims. LFM is helping rFactor2 massively in this area but still a way to go and it will never be as big as iRacing, especially if you race at odd times.
2.Ease of use/setup. RF2 can be a pig to set up, but once set up can be a thing of beauty.
3.System requirements. iRacing can run on a potato. Not sure about RF2 as I am lucky and have a top end system, but even then it needed tweaking for VR.
However, I now use RF2 way more.
1.It's better graphically on the official content ( quality varies on the free stuff, some is excellent, some not ). The stuff from the last year, especially the BTCC tracks and cars, blows iRacing out of the water when set up properly.
2.The FFB is much more engaging. I don't know if its more realistic, I've never driven a race car. But it is much more fun to drive and gives me more detailed feedback.
Cost is debatable, its all peanuts compared to my hardware costs though. I have heard good things about the RF2 LMPs though from people who have driven them, and the SCC series is proving quite popular on LFM ( multiclass with LMPs and GTEs ).
I've just found that since getting into RF2, iRacing feels a bit dull by comparison, though I'm sure I will go back to it at some point.
Hope this helps.