I'm guessing Wii U. In addition to Mario Kart coming out in 7 weeks, Wii U has four more major exclusives with 2014 release windows. I think Project Spark is the only XB1 game confirmed for release this year at the moment, and at although it has a plethora of multiplatform titles coming, it will have to contend with the PS4, PS3, 360, and PC for many of those sales (and in some cases, even the Wii U).
With the sales benchmarks you chose, XB1 will have to more than double its LTD total, and I don't see any reason to expect it to do that particularly soon. Wii U does have to sell more than half its 2013 total in the US, but with Mario Kart around the corner, we can reasonably expect some kind of boost in the early summer months.