After playing for the last week I think that Call of Duty 4 has the best of both worlds between a Halo and Rainbow Six. It has the party system that works much better then Shadow Run's. It has modern weapons and a rank system like Rainbow Six Vegas where you unlock more weapons as you level up. And the play lists allow you to have fast paced games or slower (objective, one life to live ) games.
It's coming out on the week of November 7th so that is a good month + after Halo. I really feel that this is going to be the FPS for BLS this winter.