Both are completely different games it would be retarded to compare the two. It all depends on what kind of game you like after all. But if I had to choose I would say I can't wait for GOW2 and Starcraft 2 to come out and completely change the subject.
I have to say World of Warcraft. It's very addictive and is one of the best rated and played online games in the world with over 9 million players. It's very addictive and fun. The community is pretty good overall and the minigames are great.