Agree on both of DVW's points, BF3 will blow CoD3 out of the water and into orbit.
CoD3 will probably still sell just as well (or better) for two reasons:
1. It's for more casual gamers - it's just an arcade, game, basically.
2. All the hype surrounding the brand.
BF3 is always going to be a better made, better looking and better functioning game than whatever CoD do with MW3, simply because CoD don't try to make their games compete on a serious scale - they just pack a bit of improvement onto the same engine into each new game and release it with a new campaign and boom, the brand sells.
It is designed as a cash cow; if CoD spent the same time and intricate development that went into Frostbite 2 and Battlefield 3 on each release, they wouldn't be able to perpetuate their brand and the hype, because it'd take much longer to develop each game and they wouldn't be able to sell as many new editions and expansions as they do currently, because they would actually produce a decent end-product in one package.
BF3, on the other hand, is the first major Battlefield release since Battlefield 2/2142 - for the PC, anyway - and has been pretty much developed to be the best 'modern warfare' (pardon the brand pun; I'm using this as its literal meaning) game that will set the standard for the next generation. If CoD did this, they wouldn't be able to keep selling new editions of their franchise each year.
This is only the gut feeling why I dislike CoD, though; it's not that their games are necessarily
bad; I've played them and it's still
fun to play, but when I've played CoD I've always been consciously and painfully aware of the quality deficit... There genuinely isn't anything new in the last few CoD:MW titles; they have been the same old arcade-style, small-field shoot-'em-ups as you could have bought 8 years ago, just with even more weapons and better graphics.
So sure, CoD is still a fun game to play, and it may appeal more to the more casual gamer who just plays xbox with his mates occasionally, but whichever way you look at it - or at least whichever way I look at it, as a gamer and a modder (since we of the latter group love to criticise and edit games' worse aspects), BF3 is always going to be a vastly superior game; it's going to be the benchmark for serious online FPS games for the next few years, because it's really the first of its kind on such a scale.
I can't comment on CoD's singleplayer campaign, because Battlefield traditionally didn't feature one, and I suspect the recent Battlefield titles' inclusion of a 'campaign' is an attempt to really close the gap on any attempts at criticism for CoD fanboys. For all I know, the CoD campaigns may still be superior - after all, the more simplistic and unrealistic arcade style in CoD means that it is more intended for single player enjoyment. As far as I'm concerned, however, the important factors in modern games are the scale, quality and realism of the multiplayer; because
that is, for most of us, what we buy the games for and where the true potential and full splendour of a game is truly shown, and where Battlefield has, so far at least, dominated, and BF3 promises to give a massive online experience on a truly battlefield (small 'b' for the normal use of the noun) experience, without the gimmicks, gold-plated guns and arcade limitations of CoD.
Having been in the beta, BF3 isn't
perfect; don't mistake me for a Battlefield/DICE worshipper; but even the glitchy beta which didn't feature any of the full-sized maps with vehicles was by far the best multiplayer shooter I've yet to play, and goes to show off the range of style that BF3 encompasses: which includes the fast-action close-quarter-battles that people say they prefer CoD for; but with a realistic take on it with the beautiful environmental destruction, suppressive fire, animation physics and even being glared by flashlights and blinded momentarily by laser sights.
So, yeah.