I haven't made any predictions in about a year, last time I was a complete failure and didn't even come close. I am very confident with this prediction though and decided to post.
My prediction for next gen is that Nintendo will dominate with the Wii U, Microsoft will come in a relatively distant 2nd with the 720, and Sony will fall behind and not even compete with the PS4.
My reasoning for this is pretty simple,
The Wii U will dominate Japan without question IMO. The 720, doesn't stand a chance with relatively no momentum going in. As for Sony, I think it will be very similar to the 3DS VS. the PSV there. The Wii U already has Monster Hunter, Dragon Quest is coming soon too. Nintendo also has many big franchises of their own with Kirby, the Wii series, Mario, and the more popular than ever Animal Crossing. Don't count Pikmin out either as I see that growing a lot with the Wii U. I just see it being a landslide win for Wii U in Japan. This alone will be a huge advantage.
The Wii U is off to a slow start in Europe but this will improve in time. Once the big games start coming, so will the sales. This will be close between all 3 and there will be no clear winner in Europe.
In the U.S, I see it being a battle between Wii U & 720. The Wii U will sell slightly better IMO with the 720 in a very close 2nd. Sony is losing presence in the U.S.
Overall, I believe Sony doesn't have the ability to compete next gen. Nintendo & Microsoft will dominate the casual market and only the most core Sony fans will buy the PS4. They don't have any big franchises to compete with only 1st party games and they no longer have the massive 3rd party support of the PS/PS2 days because of Microsoft. Microsoft will have Kinect 2.0 and Nintendo will have its Haptic peripheral as its secret weapon.
The reason Wii U will beat out 720 is because of Japan. They will be close in the U.S & Europe but Wii U will lead by at least 15 Million in Japan IMO