MikeB on 02 January 2009
The Xenon is a 3 core processor, the PS3 Cell has 8 processors on 1 chip. The overall design is very different, despite some similarities.
The Cell also fits very differently in the PS3 design as a whole, the Cell has low latency XDR Ram to take advantage of through a dedicated bus. The Xenon has to share higher latency main memory access with the Xenos and has to share its L2 cache with all 3 cores. The RSX depends on the Cell to unlock its full potential, the Xenon rather fights for resources with the Xenos for full potential. 
The PS3 also indirectly benefits from the 360 using PPC technology.
