BO - There's more run and gun, faster pace, overpowered weapons/explosives/killstreak awards, one man teams, and a lot more game type options...which I still like.
BC2 - Takes true aiming skills for long distance shooting while roaming around large environments, real team work to win games, a few vehicles, mostly open field levels, differ classes. You have to work for your 15 kills while BO you can do it blindfolded. The scoreboard points are not determined by mostly kills but objectives and helping your teammates via your classes. I love them both. 2 different beasts though. It's like choosing between your wife and your girlfriend.
warning: if you're new to BFBC, it may take time to get good. it's a little overwelming. BO you can good or decent after a few nights.