I think that gears 2 will take off the best for a few reasons.
1.) I work at GameStop and at the moment we have over 200 reserves for gears 2 and less then 100 for COD5.
2.) There are a lot of people who aren't going to buy COD5 because it's being made by the people that made 3 and 3 was, for the most part, a complete failure.
3.) Gears 1 was an awesome game and i've played the multiplayer for gears2 already and its freaking awesome. And there wont be nearly as much lag because they are configuring the way people connect a bit differently this time around.
My gamertag is gamer0080 if anybody ever wants to play.