In all honesty this depends. Are you discussing BF3 vs MW3 for PC or in general?
MW3 is by FAR going to be much larger on console - especially Xbox. I can't have much opinion on this as I have not really seen the game - except the preview from Xbox themselves - as well as it's a console-based game (IMO).
However, for PC, BF3 is extraordinary (alpha tested). That's pretty much all I can "say about it"... but if you look at the multiplayer alpha testing videos that are out on youtube... Reg is spot on for the most part, graphics are disgusting, and they have been working on the knife registration (which was sort of an issue in BC2). They also brought back prone for BF3, as well as making "fully functional bi-pods" for the MGs, which is a HUGE plus. Downsides are: It is going to be on Origin instead of Steam, use the PunkBuster ID system, and will be difficult to make into a highly-competitive game (ie: Starcraft II, CSS, TF2, Black Ops). Even though some people may think competitive games are not exactly a downside, it attracts a HUGE clientele to a game.
Oh... the other thing about (both) of them, is that neither have actual tools (or have not been announced) to go with them... Like CSS has SDK where you can make new maps and mods, etc... That and all the new DLC crap that the companies do annoys (at least some) people.
And... yeah... that's my opinion on the matter.
//Rant
You should add a poll to the OP. (again my opinion).
