I have both played 40 hours on BF3 and all day on mw3 so far, so probably in a good place to comment on this debate.
MW3 is just rehashed MW2 a few new game modes and streaks and weapon perks (reduced kick lol). I loved the gameplay of MW2 and the shooting mechanics, great for the runner gunner kind like me. And as it is pretty much the same I am so far again loving the general game mechanics. IWnet is still a pain in the ass, could not migrate to host e.t.c. the graphics to me feel worse than MW2 but that maybe becuase i have just spent 40 hours playing BF3. I have a big worry though the maps don't feel good. They pretty much all seem to be very congested, I have not found a map with interesting open areas. Granted it takes time for maps to click and for you to start to love them but they don't seem to have iconic maps. MW2 had great maps like Estate and Scrapyard, I also loved wastelands. Blops had weaker maps but still some good ones like villa. These maps all feel very forgettable so far and I really don't think they will grow on me.
BF3 - The 1st thing about BF3 is the graphics it is so many levels better and so is the sound (though it is hard to hear footsteps if someone is close). It has a steeper learning curve but vehicles really add a different dimension to the game. There is a lot more you can do which adds interest to the game. My problem with BF3 is struggling to find a place. I don't know if i should be playing 32 rush, 64 conquest, 24 TDM e.t.c. while this is a problem for me it is a benefit to most as you can find the game type that you enjoy and stick to it. I also feel unlocking all the attachments for every weapon is a little tedious. Finally while the team work aspect is great in concept i don't see much of it. But when I do it really elevates the game.
Before my conclusion some background to show I am not a fanboy. Quake 2, CS 07-1.4 (i think) then a big sebatical (called wow). Then Mw2 480 hours played. Black Ops 342 hours played. BFBC2 16 hours played.
Despite my recent history with COD for me BF3 is the game I want to play! Having said that I have decided to give MW3 3 days of play to see if the maps grow on me, i really don't think they will and then I will be back to BF3.