Depends on what you like.
Here are my impressions so far:
On the technical side the net code does not appear as stable as in rFactor.
They should (or at least this is what I thought) be using the same engine but I suspect by fiddling about they have gone and broke some of it.
On the artistic side it is a fantastic game!
Immersion is simply fantastic in terms of visuals and audio!
Even the cars are designed to respond to the small things. Brake temps, tyre temps, engine temp etc. This exists in rFactor aswell but it just seems that the car physics are tweaked to be more sensitive to changes in GTR2.
GTR2 just really puts you on the racetrack. You can almost smell the petrol and rubber! (At least this is how it feels to me)
But except for the rain there isn't anything in GTR2 that a good rFactor GT mod could potentially provide.
So this is essentially what you would be bying; An extremely well polished rFactor mod.
(But I really do mean _extremely_ well polished!)
Here is a forum thread setting up GTR2 vs rFactor:
http://forum.rscnet.org/showthread.php?p=3211763
In there you will probably get the general concensus (Spellchek!!?).
-"Winning is like a drug...I can't settle for second or third in no circumstances whatsoever"
(Ayrton Senna)