What's the difference in the supposed competitive and cooperative matchmaking modes? Do you simply compete for score in a sort of Borderlands-esque experience, or can you actually play against each other in teams, a-la Halo? Like, killing each other--not AI. And if you in fact can compete competitively, is there any word on whether there will be an Arena or 1-50 ranking system like Halo 2, 3, and Reach? Or do we just know nothing at this point?