I find it difficult to predict how the consoles will actually perform relative to each other without knowing things like game lineups, etc. For instance, if Sony can get more exclusive Final Fantasy, it could protect them in Japan. MS could get an exclusive FF, on the other hand, which could allow it to finally break seriously into the Japanese market.
But what interests me is the attitudes, rather than the sales numbers.
My prediction is actually one of a difference in alignment. In the 7th generation, Nintendo went its own way, while Sony and MS pretty much mirrored each other. I don't think that's going to happen again this generation.
Nintendo's path, of course, is already somewhat laid out. Their task now is to get some more exclusives and to draw more third-party multiplatform games to be best on the Wii U. But the die has already been cast, for them.
Microsoft, I think, will want to further bring PC and Xbox together. I think that their motivation for getting into the race to begin with will assert itself again, and we will see the Xbox Successor being significantly more powerful, and more expensive, with a Windows 8-like interface. I also suspect that we will see the underlying system being very similar to a PC, so much so that most Xbox Successor games will also be PC titles - MS's hope will be to make them almost synonymous, so that PC and Windows is protected.
But Sony is the interesting one. They're in an unstable financial position, meaning that they've no chance of selling their system at a huge loss as with the PS3 at the start of the 7th generation. And their experience with the high price of the PS3 at launch will force them to keep their price relatively low. As a result, I predict that the PS Successor will not be much more powerful than the Wii U - say, within a factor of 2x, comparable to the gap between the PS2 and Gamecube (but in reverse, of course). I also predict that Sony will emulate much of Nintendo's direction, with more focus on Move and other non-traditional controls.
And my main prediction is that this change in alignment in the 8th generation will result in some interesting dynamics. More games will be WiiU/PSS (Playstation Successor) or PC/XBS (Xbox Successor), rather than the 360/PS3 pairing that was most common in the 7th generation. Note that I make no prediction of which side will be more successful - that's something that is yet to be seen, although if history continues to repeat itself in this regard, the Wii U will be most successful due to its lower power (lowest-power system seems to always do best), while the XBS will struggle as developers focus more on the pair that is easier to port between. But then, it's hard to say - XBS might do best because it's the only one that developers think of as "really next-gen"... which would be sad, in my view, but I could certainly see it happening.