I play Left 4 Dead 2 almost religiously, with co-op being the best I've ever played on the 360. Since Valve ported the L4D1 content to L4D2, I've never played a game that had a higher replay value than L4D2. I mostly play on my 360. Can't beat XBL for the social aspects.
I use my PS3 only for the exclusive content. I'm a big fan of Uncharted and looking forward to The Last of Us next month.
Personally, I think PS3 is a technologically superior gaming platform, but it really falls short on the social aspects of gaming. I don't use PSN if at all possible because I really don't trust Sony. Being an engineer in the Internet industry and IT, Sony's past "network events" tell me a lot about how they run their network and systems. I really just don't trust them, which is one of the reasons why I mostly use my PS3 for single player gaming.