Well, this is how I see it...
BF3
- Up to 64 players
- Large maps
- Destructible environments
- Vehicles - AA mobile thingies (

), Tanks, jeeps, APCs, helicoptors, jets, boats
- Focus on team work, squad based play, squad members act as mobile spawn points, classes with different support abilities. -
http://bf3blog.com/battlefield-3-classes/
- When firing long range have to aim higher to counter bullet drop/realism etc.
- Item unlock system is based on your actions within game. Using certain classes progresses related unlocks. Using vehicles progresses vehicle specialisation unlocks etc.
- PRONE!
MW (not played 3, but have played a fair bit of 2)
- Up to 16 players
- Smaller scale maps
- Arcade feel, fast movement, run and gun play
- No vehicles
- Pointstreak/Kill streak bonus system
- Less focus on team play
- Item unlock most probably works like Black Ops system, you earn points and can unlock the things you want rather than just the next thing in a set unlock structure.
They are two quite different beasts. Really depends what features you like more.
I actually played more MW2 before switching over to BF2:Bad Company 2, but never went back after switching. Seemed to be more thought to what to play next. Didn't just pick your favorite weapon loadout. If your team have a bomb planted on an MCOM station and are laying down suppressing fire to prevent defuse then pick "Support" and supply them with ammo. If your team are defending a flag position and are under fire from a tank pick engineer or recon to help take it out with explosives. If your squad is pushing a front and needing to keep the pressure up, pick assault with med kits and defib and spawn on them and keep them alive.