My feeling is Nintendo will win this one easily, unless they make Wii Music the centerpiece of their conference. My reasoning is that everything is unknown at this point - so whatever they show will be new and exciting (Kid Icarus, Animal Crossing, Zelda *fingers crossed*).
Add to that a tech demo of the new MotionPlus, if done right this would make everyone's jaw drop, and especially if demoed with a new Zelda (again fingers crossed).
Add to that the inevitable third-party surprises, Capcom still has a "surprising take on a next-gen favorite" to unveil (RE5 *crosses fingers*), although it could end up being a lame game. SE could reveal KH3 or DQX. And of course we will be seeing The Conduit, which will be nice but probably won't be a showstopper.
There's also the possibility of a DS redesign and a Wii storage solution. One or two things of what I have mentioned are bound to happen, and will probably be enough to outshine Microsoft's conference, but we will have to see.
