Depends, halo 3 has amazing matchmaking but like boring campaign, so get ODST which comes with halo 3 matchmaking and a better campaign AND firefight. If you played other Halo's before, just get gears of war 2 cause it really is like the same game recycled with a little bit more especially if you played halo 2
Gears of War 2 is a really hard game to be honest, and the matchmaking isn't that great. It's an amazing game never the less. It's really hard to get good at it online. Halo is a bit easier which makes it more addicting.