this is random I know but i talked a few people in my line (ike 4 or 5 people) out of their Xbox 360 versions of the game left 4 dead for the PC version for the Steam version since the online community is greater and Steam offers a ton of other games and great deals.
Pros of 360 version - 360 online community, access to NXE features, have your entire friends ist on 360 to play a full game. OF course you have the 360 achievements.
Pros of PC version - PC community, mod support, Steam's community features, game looks better mattering hardware. Only issue is that you have to build a new friends list.
This is basically how I explained it.