I havent played BC2 myself yet, but have MW2 and have been playing it since it came out. The whole gameplay is really nice. The graphics improved quite a lot on MW2 vs MW.
There are a lot of new guns and some new kill streaks (which I think are a pain in the arse) Took me quite a long time before a got a nuke.
The maps, they are pretty poor. Really big maps and not enough players. Where MW2 turned me off the most was the match making from IW. They stuffed up the multiplayer mode big time. The dedicated servers on MW were amazing and I used to play MW in ToG Clan. But gave up clan gaming because of commitments.
Now I play MW2 for fun. It is a nice game but if you are really interested in playing multiplayer online, I would advise to stay away from MW2. If you can get your hands on MW1, play that instead