MW2
-Splitscreen
    Seeing as I constantly have company over, splitscreen is an awesome way to 
    play together.
-More fun alone
    MW2 is fun even if your team sucks and you aren't playing with friends. 
    You can still do well personally even if your entire team is a bunch of 8
    year-olds.
-Smaller Games
    I've always preferred smaller games in FPS multiplayer, and even if this one
    doesn't have the 8 players I'd prefer, its still small enough.
-Automatically Fills Games
    Even once a match has started, if someone leaves, others can still join.  
    This is probably the one thing I don't like about BC2: you can find   
    yourself in matches of 3 vs 10.
-Killstreaks
    When you're doing well, EVERYBODY knows.  Its awesome to pull down crates 
    from the sky or fire death from above from an airplane.

BC2
-Destructible Environments
    Nothing is more satisfying than toppling a tower and watching your kills  
    rank up, or blowing apart a wall to flush out those hiding behind it.
-More fun with friends
    This game is much more team-work centric than MW2, making it more fun with 
    friends.  Correlating a strong strategy and putting it into effect to 
    conquer the crates is immensely rewarding.
-Vehicles
    While I don't feel the lack of vehicles hurts MW2, they certainly make BC2 
    a hell of a lot more fun.  Even if I crash anything with wheels and drive 
    helicopters upside down (to which DA can attest ;]), they're still a ton of 
    fun xD
-Classes
    The classes let you interact with and help your teammates so much more than
    you could in MW2.  You can heal them, revive them, give them ammo, repair 
    their vehicles, and spot enemies, and then continue to work together to 
    destroy the opposing force.  BC2 is very much a team game, and that's what 
    makes it so fun :]