+1 for AC
I made the switch to AC but enjoy rf2 still. rF2 is great, but AC has more mods and a bigger community.
~1900 servers vs. ~450
~9500 mods vs. ~700
The AC physics are just as good IMHO. They both have track rubberisation, non-linear grip coefficients, Pacejka cruves (RIP Hans Pacejka), chassis flex, etc. Main difference is rf2 uses an in-house physics engine where-as AC leans on ODE for collision and rigid, articulated body simulation. I actually find rf2 to be too clean and clinical, while AC has this rough and real feeling. And it's $20 :-)
I picked up the Ultimate edition of AC for $30, I had a bit of a play with it, but something just doesn't feel right with it on the G27, maybe because I have spent so much time on PC2 recently, also I just couldn't get the buttons to map as I wanted. I will have to have another look when I have some time to get into it.