It's really tough to say. But trying to put things in perspective so we can get a clearer picture of what could happen..
On one hand, Microsoft just BARELY edged out the Gamecube, which was Nintendo's ultimate low point (excluding the horrendus Virtual Boy). But on the other hand, they've had a massive upswing with their more recent console, the Xbox 360 which doesn't quite yet suggest any turnaround carrying over to their next console, while Nintendo seems to have hit a major decline as of late from the huge boon that was the Wii. So the more recent history shows the odds in MS's favor, while the overall picture shows largely Nintendo dominance over MS.
The main issue that will hold MS back is Japan, as well as most of Europe. The price of $500 is also much less consumer friendly than their previous iterations, and is significantly higher than the Wii U, whereas Xbox and 360 were pretty similar in price compared to their Nintendo counterparts.. There is also the increasing DRM stuff MS seems to be pushing, which I think it going to create a backlash that could come back to haunt them.
I think in the end, Nintendo will maintain a VERY slight edge on Xbone, but it's going to be close, and depend on a number of factors. If, for instance, Nintendo decides to cut the life of Wii U short (shorter than the typical 5 years), which I believe they should actually, then Xbone will still likely outsell Nintendo, even with all the things going against it. But assuming a 5 year lifespan for both, I'd say there's a pretty decent chance of Wii U edging it out, at least by a few million. It will be a reverse of the Xbox vs Gamecube result, with Sony still dominating of course, though by smaller margin.