This one has actually got my curiosity.
Personally i will be going straight into co-op (I have friends through steam ready and waiting!)
I would prefer to go co-op against AI, for 2 reasons....
1 - To test just how "SMART" The AI (Bots) really is in this game!
2 - To get a better feel/knowledge of the game, To go through my setup and prepare for going against real people.
So Let me know what your answers are!