I personally can't wait for the multiplayer. The new point streak options for support and specialist makes my day. I also can't wait for the new spec ops. As much as I love animals and would never condone harming them in real life, exploding pooches look awesome as enemies.
I'll probably do the campaign a couple of times, as I do with every CoD I buy, but first week or so will be spent in multiplayer figuring out what weapons I am going to use, perks, and point streaks.
I would do both MW3 and Battlefield 3 this holiday, but battlefield games (not bad company, as in my opinion it is another separate ip) will play best on PC and I refuse to download Origin until their terms of service are changed. It's the same reason I'm going to miss out on The Old Republic.