If you have both a 360 and PS3 with both online services I would go with whichever one you have more friends that you will play with on. Me, I'm going with my 360 b/c of DLC, achievements (trust me and all other 360 users they're great), XBL is an overall better service (faster download speeds, better connections, more users, more DLC in general), no mandatory install (you will run out of space on your PS3 HDD pretty soon) the difference in graphics is less than negligible shown here
, I prefer the 360 controller a ton better, XBL is only $50 for 13 months less than one game (definitely worth it). Also if you are going to argue that PSN is cheaper I say that the 360 is cheaper. That is why I am going with the 360 version.