If I remember properly at E3 2005 :
RSX was marketed at 126 shader ops per cycle while running at 550 MHz.
Xenos was marketed at 96 shader ops per cycle.
there was almost a 25% difference in raw power.
But RSX have been clock down to 500 MHz (narrowing the gap in raw power).
Xenos seems to make better use of its execution units, it keeps them more busy if I understand properly,shortly it's more efficient.
It's better at branching.
Has a edge in regard to functionalities.
More if we trust Joker 454 xenos performs better than rsx even in pixel shader heavy situations.
The xenos is less bandwith limited too.
For me it's an overall (sorry for NAo and Fran who went in terribly long explanations about this, I understand that things change depending of the task
) slightly better gpu, no matter how hard some are trying to spin it ( I don't speak of Nao, Fran or others like Farid, etc. who just explain that nothing can be better every time under every workload especially when we speak about state of the art gpu (except g8O))
A lot of devs have stated that the ps3 is a superior system because if used properly the cell outperform the xenon from 2 times to x times depending on the task.