I think the Xbox 720 will sell more than the PS4 and the Wii U combined. The reasons for this prediction are numerous...
1.) The Wii U's design is difficult for devs and so it will recieve little 3rd party support in the short run, and even less 3rd party support in the long run. Due to this lack of 3rd party support, sales will be low, and will continue to be low. This viscious circle of lack of 3rd party support and subsequent lower sales will cause the Wii U to have significantly lower sales than its predecessor.
2.) The PS4 will not have the same percieved (by the general public) graphical advantage that it had over the 360 this gen, as they will release closely to each other (and therefore have similar specs). Also, if Sony postpone the release of the PS4 in order to get a graphical advantage over the 720 then this will be hugely detrimental to sales as it will give time for the 720 to establish itself in the market, so no matter which strategy they choose, they can't have the same benefit that they had in the previous gen.
3.) Tablets and smartphones are the future, and this is because consumers are looking for as much of their entertainment requirements as possible packed into one device. The Xbox 360 is currently building up a large entertainment package with apps, music, games, movies etc. and im sure they will continue this strategy into the next gen.
4.) One of the main selling points of the PS3 was that it had internet browsing capabilities, and in the next gen both the 720 and the PS4 will come with internet browsers, but the 720's will be superior if the new IE from microsoft is all it's cracked up to be.
5.) The main selling point of the PS3 was that you didn't have to pay for online gaming like you did with the 360. Next gen though, I suspect that Sony will introduce an online fee, as they cannot repeat the same mistakes they did with the PS3 and sustain $10bn losses. This will further shift the market in support of the 720.
6.) The Xbox 720 will more likely launch at lower price point than the PS4, and with all the advantages the 720 will have over the PS4, consumers will likely choose to purchase the 720.
7.) As companies, both Nintendo and Sony are struggling financially and so wont have the same budgetary capabilities as Microsoft have in terms of marketing.
8.) The Wii U lacks the same gimmick that the Wii had during it's launch period. When the Wii was released the controls were new and exciting and many people purchased Wii's to experience this innovation. However, the Wii U is launching with tablet technology, which is already established in the market, and so the Wii U will not benefit for the "curiosity" sales which the Wii had.
9.) I think people will begin to get tired of the constant stream of Mario games being released, and I think the series will experience a significant decline in sales. This will cause the Wii U to lose one of its major selling points.