I said this onthe PS4 thread, but I think Sony will have the weakest of the three next gen consoles because they were always the most successful when they had the weakest console like the PS 1 and 2. This time around, Microsofot will have the most powerful of the three and Nintendo will be in a close second to Microsoft with the Wii U.
The PS1 and PS2 weren't the most successful because they were the weakest hardware. They were the most successful because they had the most 3rd party support, which they got by default in those respective generations because Nintendo and Sega dropped the ball. In fact, the PS1 and PS2 weren't even the weakest hardware. Remember the Dreamcast? The Dreamcast was the first console of the 6th generation and it was the weakest. So if being the weakest console had anything to do with a console's success then by that logic the Dreamcast should have been the number 1 console, but that wasn't the case.
And as for the PS1, I'm not sure it was the weakest system then either. The Sega Saturn might have been weaker in some respects, and there was also other consoles of that generation such as the 3DO and Jaguar which may have been weaker too, but I'm not certain on that. But the point is how weak or strong a console is isn't really the most important factor. Software support is probably the biggest thing.
Being first on the market and getting a head start is also an important factor. Since it looks like the PS4 is going to be the last one on the market there's a good chance they will too far behind to ever catch up to the Wii U or the NeXbox420. I do think they will be the most powerful though, which is the advantage they get by having an extra year to develop and finalize the technology of their system. Being the most powerful system probably won't be enough to overcome the disadvantage of being last to launch, but it is certainly better than suffering the double whammy of both being the last and the weakest.