XCX>XC3D>XC
I think the N3DS port will get good sales for three main reasons. One, it's the only N3DS exclusive currently announced and a lot of people who own a N3DS will likely be interested in a game that pushes the system. Two, loyal XC fans who have a N3DS will likely purchase it. Three, the game has a lot of positivity from reviews and word of mouth as well as heightened exposure from Smash Bros. I think to can combine to bring in at least 1mil sales.
However, Xenoblade Chronicles X has a number of things going for it the original just doesn't. Most obvious is that it is a lot better looking, which goes a long way in promotion. Another is that, as a whole, it's much more in line with the mainstream. It's open world obviously. It has online co-op. The aesthetic is much more suited to mainstream marketing while still being distinct. And of course, this game has much more backing from Nintendo as a whole in the west. It has been in the spotlight a lot more than the first game ever was. And all things point to a much better handling of distribution as well as proper promotion. And another thing is timing. The first game launched on a console with 100mil users, yes, but during a time when the console was viewed as near dead with nothing to offer the core gamer. In short, the lowest point of the Wii in terms of image. Here, it's the opposite: the Wii U may have a small install base, but the view of the Wii U in general is far more positive and people are interested to see what's coming. All these combined, I think Xenoblade Chronicles X will cross 1mil without any trouble and outsell the 3DS game.