My biased opinion is this; MW2.5...sorry MW3 will have the exact same animations as its predecessor MW2. It will still use the quake engine from 1999. The developers have spoken out that they aren't improving the engine and thus not the graphics, thus not the map size, and therefore the gameplay will most likely stay the same. Battlefield 3 on the other hand IS using a brand new engine called frostbite 2.0, featuring a light system capable of doing an almost infinite amount of lightsources in a single object. One object in BF3 has more lightning detail than an entire map in BC2. Also if you have played the FIFA series you know that they have an superior animation system in every aspect. this ANT system that it is called is integrated in Frostbite 2.0 allowing for life like animations with real weight added to it. Something most games today lack completely (Im looking at you CoD 7,6,5,4 and your flying on ground running..).
Did i mention that BF3 with FB2.0 will deliver support for 64 players, but DICE playtested with 256 players?
And I forgot vehicles!
And the most important thing, destruction 3.0 allowing to wipe out any enemy cover you can find o.O.
Also the Battlefield series have always had superior sound technology to the CoD series.