I decided to go for the PS3 over the 360 version. The 360 typically has slightly higher texture resolution (at times) and better anti-aliasing. Although I prefer the FF wheel compatibility and free online multiplayer on the PS3 over the graphical advantages the 360 version has.
People's situations will vary though. I have a decent wheel (G27) & racing rig set-up (VR3) so it makes sense to use the PS3. If you're just planning on playing with a controller and already have XBL Gold, the 360 version would be the way to go IMO.
I can't decide whether to commit to the GAF league either
I'd like to, I'm just more of a single player gamer. Playing PGR2 'cleanly' with a decent group of players is one of my fondest gaming memories though. It sounds like the GAF league respects racing etiquette, so the interest in joining is definitely there!