Well, if you're on PC, the steer away from MW2 because it doesn't use dedicated servers which means, unless you are the server, you will experience major lag. Also, because it's not dedicated, cheating is rampant on the PC version of MW2.
BF: BC2 however uses dedicated servers. Therefore, you can find servers that can be monitored and will boot
I haven't played MW2 but I have played COD4. The game play between BF and COD are completely different. I would say get BF if you like multiplay the best but get COD if you like the single player side.
For multiplayer the maps are completely different. COD maps are tiny in comparison to BF maps. There are pros and cons to each one so its up to what kind of gameplay you like. The bigger maps in BF allow you to spread out more and slows the game play but also pushes you to work closer with your teammates. COD maps make for quicker paced game play and you can run around more solo style.
I only have BFBC2 and I'm having enjoyable time in multiplayer matches.
Also, the maps are free, LOL.
Yeah multiplayer is the meat and potatoes of all BF games. The single player looks great and all in BC2 but the game play and skill level require just doesn't compare to what you experience playing online with real people. I think the main difference between COD and BF is the squad organization/communication. When I was playing COD4 for a while that was the main thing that I missed that had me shutting down the game and firing up BF.
What sucks about BC2 is that it is a dumbed down version of BF2 that makes it more of a run n gun game like COD. I miss having the squad leader role because it added a lot more depth to the squad play. You had to work as a squad to not only capture points but to keep the squad leader alive so you could spawn on them. They took out the commander role (which I don't mind) but along with it we lost the UAV and air strike mechanics. Allowing all of the snipers to call air strikes is a little too powerful I think but overall, I still enjoy the game.
I completely agree with SpinachEater. BC2 is leaps and bounds better than MW2. Another major difference between BC2 and MW2 is that you can't ace an entire time in BC2 (unless the team is tiny), which forces you to work in teams (squads).
But SpinachEater is right about BC2 being closer to MW2 than BF2 is to MW2. Though the mechanics are very similar, the maps in BC2 are a bit smaller that BF2 and the player map sizes are a bit smaller as well (no more 64 player maps D= ).
BC2 is a bit more accurate in the realism realm where you can destroy the buildings and landscape (to a certain extent) and the ghillie suit (sniper camouflage) actually does work quite well. BC2 does not support proning like BF2 and MW2 does but proning I think leads to the dolphin divers that run rampant in BF2 and quite frankly BC2 doesn't need proning. As some other people have mentioned, there is bullet drop and leading required in sniping at longer ranges (no wind however). The bullet drop also extends to tanks and their shells.
I will say that MW2's single player is a bit better than BC2's single player story but bother are quite well. Both games have their focus on the multiplayer aspect more so than the single player aspect.
BC2 does have more weapons and allows for customization, as well as more graphically appealing compared to MW2. There will be one thing that might get on your nerves or at least gets on my nerves and that is the DUST. It's everywhere. After you blow a hole into a building, after calling a mortar strike, after shooting a tree, using a grenade, etc etc.
That said, playing BC2 on high settings will require a decent computer due to the increased graphics requirement.