Xbox 360 vs. PS3 - Comparison by Microsoft

You should of course understand that this article has been written and made by Microsoft and not by an independent source or so - but it can still be an interesting read nonetheless.
Here's the summary:

There are three critical performance aspects of a console:
- Central Processing Unit (CPU) performance:

The Xbox 360 CPU architecture has three times the general purpose processing power of the Cell. Cell's claimed advantage is on streaming floating point work which is done on its seven DSP processors.

- Graphics Processing Unit (GPU) performance:
The Xbox 360 GPU design is more flexible and it has more processing power than the PS3 GPU. In addition, its innovated features contribute to overall rendering performance.

- Memory System Bandwidth:
Xbox 360 has 278.4 GB/s of memory system bandwidth. The PS3 has less than one-fifth of Xbox 360's (48 GB/s) of total memory system bandwidth.

When you break down the numbers, Xbox 360 has provably more performance than PS3. Keep in mind that Sony has a track record of over promising and under delivering on technical performance. The truth is that both systems pack a lot of power for high definition games and entertainment.

However, hardware performance, while important, is only a third of the puzzle. Xbox 360 is a fusion of hardware, software and services. Without the software and services to power it, even the most powerful hardware becomes inconsequential. Xbox 360 games—by leveraging cutting-edge hardware, software, and services—will outperform the PlayStation 3.