This is based on these head-to-head comparisons in an article from IGN that I found interesting. To me this is like apples and oranges. Gears and Halo play entirely different, and while similar in some respects are almost different genres.
Here's what THEY said at IGN-
Graphics- GoW2
Halo 3 came out over a year ago. Of course Gears looks better.
Sound- Halo 3
I love the Halo themes that pipe in as the action goes into overload.
Gameplay- Halo 3
GoW is close, but I feel that it's stop/go kinda action isn't as frenzied as the combat in Halo. Halo offered many new options and enemy types. I also think in the campaign mode the AI is much tougher in Halo than Gears, even with ramped up diffculties. However, NOTHING is as thrilling as tearing through Locust with a chainsaw bayonet!
Story- GoW2
I disagree with this but I understand why they felt that way. Gears feels a little more personal, while I think Halo thrusts you into an overall larger conflict on a galactic scale.
Features- Halo 3
Halo 3 has tons of great features, the Forge and 4 Player Coop. Horde mode in Gears is great fun.
Multiplayer- Halo 3
Halo offers a ton of online content, more modes, maps, and variety. They aren't even finished with the new expansion pack coming in a couple of months.
Text Review-
http://xbox360.ign.com/articles/934/934644p1.html
Video Review-
http://media.xbox360.ign.com/media/1...80/vids_1.html
Again, I feel it's like comparing Ghost Recon-Advanced Warfighter to Call of Duty. They cannot accurately be compared because the overall experience is totally different. Plus one game is now older, though still fun as hell.
They are both great in what they offer, and two of the best shooters ever.