i play the game for the fun of the online multiplayer stuff and nothing else the regular game mode I only play if the PSN is down or if my net is out. So I could careless where or when it is set.
Where and when it's set is pretty much the basis for everything in the multiplayer though. Different era = different guns, equipment, maps, kill streaks, etc. Obviously you'll most likely be able to find guns and maps you like regardless of setting, but it has a pretty big effect on everything.