Wow.. another close call this one will be..
i think its almost a 50/50 call as we really have no idea how it will launch in america/europe, since there isn't really a fair basis of comparison (wii userbase is different to GC userbase)
I'm going to say Halo 3 still > SSBB after 2 weeks, but with Japans two weeks, America and Europe only need about 3.4mil total over the 2 weeks.. its very possible