MW3:
>Basically same engine as last Modern Warfare games
>Generally same game play for multiplayer and singleplayer
>Everything is more shiny and some guns have new looks
>Some new co-op and multiplayer game modes
>Still has kill streaks, although they handle it differently in MW3
>More of an arcade shooter
>Basically the same thing since the last two Modern Warfare games
BF3:
>Brand new engine
>Destructible environments
>More focused on team based gameplay
>Instead of kill streaks you can drive vehicles or man different positions in said vehicles, i.e. You have a guy that drives the tank and controls the main cannon, and also someone who mans the machine gun on top
>Graphics about as real as you can get
>64 online multiplayer for PC, 24 for consoles (mainly because consoles are operating on older technology. Battlefield was a PC only game for a while anyway.
>Matches take much longer due to the fact that the maps are so much larger than they are on Call of Duty, which allows for all the tanks, apc's helicopters and jets
I've probably forgotten a bit but this is basically what you need to know