From the way things look you most probably will play as the arbiter in co-op.This isnt really that bad except for that god-awfull armor hes wearing, i mean does any1 even like it. Bungie whants to make co-op with both of them ok, after the first level they take away the arbiter armor and give him the golden comander armor from the beggining of Halo2. Which is obviously way kick @$$. Im sorry i like the arbiter except for his armor.

CoD4 its trailer looked great and showed gameplay and change, to its formula of run&gun. Halo only showed tiny clips of singleplayer but no real change to whats been done before. Ill probably get both CoD4 first then Halo, simply because they actually tried something new and it looks awesome.