Two things, no, nothing to suggest the PS4 of next Xbox will be ahead, if they end up a year apart on release they could be a year apart on graphics generation as well, process/power/etc. Also being AMD doesn't mean an awful lot in terms of being similar, you can do vastly different things and AMD has never had a problem creating new(ish) IP for customers. 360 and Wii have AMD gpu's in them, that are nothing alike on power or architecture really.
Why didn't Sony turn to Intel, the next 2-3 years are going to be HUGE, chip stacking, memory on die, insane bandwidth to local stacked mem, new interconnects between parts, interposers. If you go Intel chip and AMD gpu... basically no chance for ANY of that to be included, you'd get a Intel chip on one package and an AMD gpu on another. AMD Gpu and IBM/ARM/AMD cpu and you can start working together, or more closely and making some chips with serious special source.
It's starting to look like new xbox quite early, good bump in power but nothing truly "out there", while PS4 could still be 2 years away, but could have a huge amount of "special source" thrown in, AMD cpu/gpu stacked, with memory, some new interconnect, insane memory bandwidth for the GPU, vastly less traces, cheaper pcb, cheaper console(for the power).
Xbox could essentially be "pre revolution" and PS4 could be "post revolution".
Ps3 did some good stuff, bluray was a great edition as its considered massively better as a home entertainment system than just a console compared to the Xbox, but as it was so much later it really could/should have been able to come with a better GPU and Cell likely held it back badly(time to market, cost).
As for Cell, its dead because it gained traction nowhere else. IBM's plan was always Sony as a major cash influx to get Cell up and running, then scaling it down to fit in every other Sony device that gets made, tv's, bluray players and selling it to other electronic's manufacturers. But Cell basically didn't end up anywhere else or remotely useful. It's like AMD/Nvidia, you spend hundreds of millions on R&D to get the best high end graphics, but you subsidise the R&D spending by selling millions of midrange and 10's of millions of low end gpu's to "make up the numbers". Cell did the high end, failed miserably in making up the numbers and has got itself canned. Sony can't subsidise the R&D with Playstation sales only so a vastly cheaper alternative is what they are going for.
A standard AMD chip won't necessarily be the absolute best for gaming that they could produce, but could cost them hundreds of millions less in R&D.... an easy tradeoff to make