I see 'single player' as against other 'single players', Not as playing against a bot. Summing up why I don't like multiplayer games as much, I don't want to screw up my teammates, And I have friends who will shoot members of the team.
Honestly, I love both single and multi-player games but I play Garry's mod - TTT a lot and that is online so I am voting for Multi-Player
I love them both, but it's much better to compete online and shoot at real people instead of bots. ^_^