MW2 was fun for me in the start but becomes repetitive later on, it's just a simple run and gun game which can be fun.
BC2 on the other side is really something new, especially if you find yourself a nice squad with people with headsets. This game requires a lot more teamwork and has more depth too it. Also the sound effects are just amazing and really suck you into the game.
BC2 is by far the more fun game to play over the long term also because the maps are bigger and more fun and each game is a lot different from the other.
Oh and Black Ops is gonna suck ass, just another CoD game with the same stupid perks, same sort of weapons with different names and still the same small maps run and gun bullshit.