Neither, both games are lame. Games intended to be realistic like CoD just don't do it for me, and Halo is like the WoW of FPS games when compared to older HL mods and the likes.
As far as combat goes, no game has even touched Science and Industry as far as the individual skill involved. The game failed miserably because the only people who didn't get absolutely slaughtered were HLDM guys who were already familiar with the movement and weapon switching aspects of the game.
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As in respect to teamwork, Tribes is where it is at. All three of these videos are sick:
http://www.youtube.com/watch?v=XUVU9fWNAKs&feature=fvw
http://www.youtube.com/watch?v=i2et3cKlSSk
http://www.youtube.com/watch?v=XUVU9fWNAKs&feature=fvw
No modern game has touched either of those games in regard to game play imo. Same goes for TFC and HLDM.