The NX will outsell the Wii U for one reason huge reason. The NX is the successor to the 3DS, as well as the Wii U. A lot of Wii U games such as Mario Maker, 3D World, New Super Luigi U, and Tropical Freeze, are better suited to a handheld. The teams working on exclusive 3DS titles have done a lot better job than the teams working on the Wii U games. Luigi's Mansion, Animal Crossing, Fire Emblem, Poke'Mon and Link Between Worlds were all great 3DS games. The Wii U on the other hand has had dialed in disappointing renditions of franchises (at least for a console). The third party support for the 3DS includes Square Enix, Capcom, Level 5, and Atlus.
So long as there is a handheld model of the NX, for $200, that plays 90% of all NX games, then the NX will blow the Wii U out of the water.
Nintendo could screw it up though. They are already launching the NX in March (with Zelda as the only good launch game), which shows that they haven't learned from the terrible 3DS launch. They could overprice the NX at launch the same way they did with the Wii U and 3DS. They could have a terrible software drought early on, the same way that the 3DS had a terrible lineup of games until 2013 added all the good 3DS exclusives.