The Cell is a faster CPU, all things considered. That doesn't mean "PS3 games are faster than X360 games" -- that's dependant on so many factors, as to make CPU speed almost immaterial, unless its *really* different... which it isn't, unless you're running a relatively pure mathematical simulation... in which case the Cell is basically untouchable. Games... are not pure mathematics.
The Xenon is easier to develop for, and not really much slower than the Cell, as far as general 3rd party game development is considered... at this point anyway.
Faster game does NOT mean Faster CPU or GPU, except in extreme cases (think "1st-party exclusives"). It tends to mean "better dev team" or "more support for one platform over another".
To answer the question posed, however... which one is "better" -- that's totally dependant on what you're trying to do, what resources you have, what your schedule is, etc. etc. etc. That question is totally unanswerable.
