They're both shooter games but they're both very different. I think that the Halo games are a lot more frantic and fast paced compared to Gears of War which is what makes Halo 2 my favorite online game. Gears of War obviously had a better single player and co-op experience (online co-op also a huge plus) but Halo 3 plans on improving on all that with an epic storyline, massive levels, 4 player co-op over xbox live, the ability to watch the levels over again and detach the camera to search the level or take screenshots, and more.
Also, the multiplayer is getting some awesome additions with new weapons, new levels, new vehicles (including a massive one that can only be played on a certain huge level) and a new mode called Forge where players, with a simple click of a button, can turn into a Guilty Spark 343 (the annoying little know-it-all that followed you around in the other Halos) and edit the map, spawn weapons/vehicles, change spawn points and more, all while a game between normal players is going on!!
So in my opinion, Gears of War can't really compete until it comes up with a sequel.
