CoD MW2 - So far it seems awesome even though I don't know that much about it.
Halo ODST (or recon whatever it is called now) seems exactly the same as all the other Halo games. You will probably still have a shield, a health bar and somehow be able to pick up things that aren't humanly possible. Only difference is that they will say you are a soldier rather than a spartan.
CoD MW2 wins not because I know it is good but because Halo will probably be bad.