January through March will certainly be interesting for both consoles.
With PS4, it's hard to say how much momentum they really have at the moment because Q4 is always about price. I'm curious to see how PS4 Slim + Pro does on a normal basis. With Q1 in particular, there's a lot of great Japanese support finally arriving in mass so I wonder if that segment of the PlayStation base already bought a system or if there are some still waiting who will finally jump in.
With Xbox, to some extent I do think they turned a corner for several reasons. First, I have no data to support this but I wonder just how many existing Xbox One owners bought the S because of how much better it is of a system compared to the OG. In other words, out of all the momentum they've gained since summer, how many of those are new players versus existing players? We assumed this would happen with the PS4 Pro but why not with the Xbox One S? I don't think it's beyond reason that there's a passionate segment of the Xbox base that's willing to buy an Xbox One S last year and still get the Scorpio this year.
Second, I'll echo what others have suggested that backwards compatibility may play some role in getting new Xbox owners on board, particularly because it's supposed to carry over some game/achievement progress (is that right?). Third, Gaf likes to makes the bluray player out to be a big deal and while I don't think it really matters in the grand scheme of things, I don't doubt that there are still some people out there who saw that as an added benefit to get the Xbox One S over the PS4 Pro. And finally, Xbox had some big exclusives and marketing for AAA third-party games that may have helped it as well.
In summary, I don't think there's just one reason why Xbox is doing comparatively better at the end of 2016 compared to the months prior -- there could be multiple reasons. What happens over the next couple of months could help explain what we're seeing now though once we get out of the "new" and holiday phase. On the other hand, it'll be interesting to see what affect the wave of Japanese support provides to PS4 hardware levels over the next few months.