here is a pretty good analysis of the situation
It's difficult to make any blanket statements about the PS3 and 360 in terms of performance.
However, when it comes to PS3 exclusives, there is a fairly significant difference.
The PS3 architecture lends itself to rendering multiple frames in parallel. The SPUs can certainly help out with rendering, but what few people realise is that this often occurs in parallel with RSX.
For example, while frame X+1 is rendering, the post processing for frame X is occurring on the SPUs. This form of parallelism fundamentally cannot be done on the 360 and is simply not needed. This is clearly very hard to implement when writing a multiplatform engine.
On top of that, the SPUs are often used to do vertex processing for the RSX (Well, you basically have to). This actually takes a huge chunk of total SPU time (I believe it's around 40% for uncharted 2). Combine that with other tasks (shader constant patching, etc) and you have be very very careful in how a rendering architecture is setup to prevent stalls.
Naturally, if you are only working with the PS3 you can tailor to it a lot more.
On the other hand, the 360 has issues with memory latency. The GPU has a fairly small texture cache - and if you overrun it, the hit is usually pretty huge. Things like that. But it's more likely to get good performance from 'dumb' code. (I hesitate to use that word, as it's still very difficult thing to do - ps: anyone who utters the term 'lazy' doesn't know what they are talking about)
At the end of the day, I can bet you that some exceptionally smart people worked exceptionally hard for an exceptionally long time getting both the PS3 and 360 looking and running as good as they do. Some people forget just how old and crappy both systems are
