Well for now definitely 360 version of MW2 over all. This thing should easily be at least 6 million by the end of the year if I had to take a guess. Lowest possible of 5 million and highest possible of 7 million.
Now second place is a little tougher but I don't think New Super Mario Bros should have too many issues beating MW2 on PS3. Mainly due to Japan. But this is of course going to depend on how big NSMB is in Japan. If it does a million by the end of the year in Japan then no biggie. At least 2-2.5 million for NSMB in America and another 1-1.5 million in Europe should put sales at about 4-5 million. I'd put PS3 MW2 sales at about 3.5-4.5 million by the end of the year.
Now of course it's still difficult to tell at this stage simply because we don't know how big of a mainstream title NSMB can be... especially in Europe. NSMB could have upwards of 2-3 million in E/O if it really does take off as the big premier title. Of course it could share it with WSR, Wii Fit Plus, and Mario and Sonic. In America, it will definitely be the big title and could potentially push it over 3 million as well.
But I still think the deciding point comes to Japan. Wii games have struggled in Japan all year, aside from MH3. If it is big in Japan, then NSMB will beat MW2 on PS3 no doubt and at least give it a good run on the 360 version.
But for now, I'm sticking with this order:
1. MW2 360
2. NSMB
3. MW2 PS3
But there are always many surprises to come so let's just wait and see.
