X1 will stay ahead of the original Xbox, the 360 will be tougher to overcome with that Halo 3 spike in September
it depends quite a bit on how X1 can hold up during the lull months April to July - from what I see Witcher 3 and Arkham Knight could be the strongest sales incentives during that period
the holiday spike should be in favor of the X1 with Halo5