It's unlikely that there will be a substantial difference in power between the two next gen, just as there wasn't this gen. It's also unlikely that it'll really matter, just as it hasn't this gen. And given that both companies suffered significant losses as a result of their hardware strategies this gen, particularly Sony, which probably won't ever be in the black on PS3.
But perhaps more importantly, there doesn't *need* to be a tremendous leap in power for the coming generation. Very few people are complaining about the visual quality of games such as Uncharted 3, Gears of War 3, etc. We've finally reached the point of "good enough" graphics for most people. So where does that leave the next generation?
Simple: refinement. They should evolve the existing tech; Cell and Xenon with more cores, updated architecture to hopefully support features for improved AI. More recent graphics chips; DirectX 11 compliant for Xbox, current OpenGL nVidia for PS. More RAM--say, 2-4GB. Increase system bus to 256 bit from 128 bit. Refine the controllers (though 15 years in, I doubt Sony will change much with the DS design).
By doing this, they'll retain backward compatibility with current games, keep hardware design and construction costs low (we don't need another $5-600 launch debacle), allow developers to keep leveraging the tools and techniques they've developed and refined over the last 6 years, and keep compatibility with all of their existing software and services.
Meanwhile, they can reasonably drop the existing systems to $149 or less and support them during the transition period, just as Sony already did with PS2 while PS3 struggled to gain a foothold.
At the end of the day, though, there's an interesting truth at work here: MS and Sony have been forced by Nintendo to at least consider some action. There's no guarantee we'll see PS4 and Xbox 420 announced this year--but there's a pretty decent chance.
