Im pretty sure its 15 maps, not 15 map packs...

Also,

Finished MW 3's campaign a few hours ago. From start to finish, it blows BF 3's campaign away. The only mission I didn't like was the Russian airplane thing, Harkov holding a pistol with one hand looks goofy to the point that I thought I was playing a Treyarch game.
Other than that, much better than BF 3. Like I've always been saying, wasted potential on DICE's part.
IW makes sure you pretty much got all your bases covered before you play online. You get to use Predator missiles, the AC130 and a Juggernaut. BF 3 gives you a tank and leaves you hanging. Not only that, but they shove it in your face with on-rails jet action.

Not sure about length, but I felt the MW 3 campaign was bigger. They may be on par though.
BF 3's Achievements are somewhat entertaining, but feel like a chore at times. There's one I really did enjoy though, which was taking out the 4 lights. Having to take into account that the bullets travel more distance and changing your shot accordingly is pretty awesome.
If MW 3 is anything like MW and MW 3, we have our winner.

Also, I did not care for the characters in BF 3 one bit. MW 3 wins again. Obviously IW have a much easier time establishing that connection because this is a trilogy and this is the last game, but BF 3 doesn't even lay foundations. You're this American guy, then you're this Russian guy.
Switch back and forth, but they're both pretty useless.

Another big issue in BF 3 is how the QTEs throw you off. MW, MW 2 and MW 3 have on-rails sections that allow you to think that you're in control, very small touches and stuff like that. Being able to move the camera around when your character is interacting with someone/something is a big plus. One button presses to plant C4, to breach a room or to pick up an NPC that you're guiding yourself, instead of pushing A 4 times to help out a guy that the AI decides where to take... You feel like you're in control. BF 3 feels rigid and uninspired. They don't even pretend to let you control the character. Both final missions have the same objective and QTEs, but MW 3 manages to do it right and BF 3 doesn't.

If BF 3 had a SP on par with MW 3, I would love it do death. It doesn't, and if DICE doesn't learn, it will never have it.

For Online though, hands-down BF 3. There's not even a contest.