Gears of War 2: bigger and more badass. Bigger baddies, bigger weapons, bigger locations, bigger events, bigger story, and more badass.
I’m currently playing through a campaign co-op romp, and about halfway through. Gears 2 is a great co-op experience game. You can play the Campaign through with a friend (highly recommended over an AI buddy + revives), and through Horde mode with a party total of 5. The campaign co-op is very much the same as Gears 1. In fact, if you take the core mechanics of the game, the controls and combat, Gears 2 plays very similarly to Gears 1. The differences that makes Gears 2 stand out in the campaign are easily the set piece levels.