The cell is far more powerful. You don't hear any stories about 360s being linked together to act as budget super computers (there are several instances of the PS3s being used that way). The PS3 is way more then 10% more powerful, its just a question of harnessing the power.
The PS3 is a lot like a 8 CPU computer. It can run a lot of threads simultaneously and create huge gains in power (compare a 3.0 Ghz Pentium 4 with a 3.0 ghz core 2 duo for instance) but only if its utilized. Obviously splitting tasks between 8 processors is harder then just leaving the task to be done by one processor, but if you do it the results are vastly superior.
Of course games need to be designed for the PS3 for them to look better. A great example of the difference between using the PS3 like a 360 and using it as a multi core system is found in comparing PS3 only games like MGS4 or Uncharted with multiplatform games on the PS3. The PS3 only games use the cell more efficiently and look loads better. Square has talked about being able to render CG in real time using the PS3, which doesn't seem too far off from the truth.
The cell processor is the most powerful processor on the market and is used in many new and upcoming super computers. It is also the hardest to design things for since it requires a vastly different outlook. As games increasingly show though, its worth learning to work with the new distributed computing paradigm since that is clearly the future (look at videos of intel demoing 8 CPU 0 GPU systems that also render CGI in real time.)
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