I go back and forth between them, cause I play SWAT, but I have been told they are making a SWAT game for Halo 3.
I believe the main problem with it is the malee system, and unfortunately with the moders.
It was smart of them to add the XP system for ranking, which made leaving a game detremental to your states. But now that everyone (who had it from the start) has a star next to their name, the drop outs have started. Thus leaving you a man down or more, and in Halo that can mean a loss.
I am as big a Halo fan as anyone, and to this day believe the story and the game play of Halo C.E. is the best game made. It is why I started to play FPS, before that I hated them.