I doubt either will happen, but Wii U ahead in Japan is much more likely. It already has a ~1M lead, and while weekly sales are higher for the PS4, it's still going to take awhile for it to pass the Wii U.
Not sure why some think the XBO passing the PS4 in the US is more likely. I think that ship has sailed. The PS4 has a ~700K lead in the US, and is currently outselling (and not barely outselling) a cheaper XBO that now comes standard with 2 free games. And if positions on Amazon are anything to go by, when we get the NPD numbers for March, the PS4 may have a ~850K-900K lead. Later his year, the PS4 is getting a price cut, which if it is a full $100, is going to completely crush XBO sales from now on. Even if it's just a $50 cut, the PS4 will still see a nice boost for the following months.
