As many here have already said X1 above PS4 in US is more likely due to MS aggressive strategies (cutting down price over and over again, doing bundles with two or more games, buying timed exclusivity for big games and doing crazy disscounts), but again as many have said i dont belive either of them will outsell the PS4 in their respective regions.
PS4 still has cards to play on the US like Price cuts and all of its big games that havent been released yet, aside from that the PS systems always get supported for more time than its competitors (you only need to look at the current weekly sales of the PS3 versus the sales of the 360 and Wii, and unlike the PS3 i expect the PS4 to sell much better on the US in its later year even after the gen ends), so even if its a neck and neck race in the US between the PS4 and X1, i expect the PS4 to win in life time sales just because the PS consoles normally have better legs than its competitors.
