Banjo will pull Nintendorks over to MS's camp.
Well, i think that Banjo is important because while MS has gained mindshare with the xbox brand, they havent done as good a job with their characters/mascots.
Who is the xbox 'guy' (or gal)? Its one thing to like a brand but, and Nintendo does this very effectively, to like people (or characters as it were) is something completely different and very powerful imo. If MS can somehow get some brand recognition going with Banjo it might be very helpful in this regard. (Actually they may have waited this long so they could let the Nintendo association fade away...)
Thats why the Halo franchise expansion gets my vote. Halo is something undeniably "Xbox" and the more it permeates pop culture the more people know the xbox brand via association.