I'll always buy the 360 version of a cross-platform game for the following reasons...
1.) 360 has a far superior controller (IMO).
2.) All my friends have a 360. Very few have a PS3. If a game has multiplayer or co-op, I'm going to get the 360 version for that reason.
3.) Achievements add to the overall experience. I always laugh at the "e-penis" jokes here on DP. It's such a transparent and silly arguement. I couldn't care less what other people think about my gamerscore. I've had a lot of fun going for certain achievements over the past 3 years and that's what matters in the end. Achievements are a small "reward" for challenging yourself.