I don't know, I've not played rFactor. But on the racing boards I read on of the few guys that have tried sims most I've seen prefer iRacing. The developer Dave Kaemar has a long history with some famous car sims and online gaming. The iRacing team seems to be focusing the model on proper physics and spending time trying to create a series type atmosphere.Ron, definitely check out rFactor if you get the chance. I think it might be a superior simulation from what i have read.
rFactor is far more popular for sure. Folks can set up their on servers and so on so there is no end to action it appears. And cars available. I won't be able to say which is "better" since I've not tried rFactor, just reporting back my experiences thus far.
And thus far I like driving the sim (I'm sure I'd like rFactor too) but it ain't no real car. No way I'd stuff a Solistace at VIR like I did on the sim. Still, pretty cool.