The PS3 is definitely the more powerful console. However, very rarely will multi-platform games look better on the PS3 than the 360. Cell (the CPU of the PS3) is extremely difficult to develop for. This is only worse now that development costs are very high. Not to mention that Xenos is quite a bit more powerful than RSX. The RSX is essentially just an optimized GeForce 7800, while the Xenos and Xenon are designed to work well together. Very rarely will the high capacity of a Bluray disc actually be useful. It can allow less sound or texture compression, but very rarely will that have a noticeable in-game effect. The reason for for 360 games performing better than their ps3 is because of the simplicity of developing for the 360. Games are developed on the 360 first, and then ported to the PS3. However, if both games are built from the ground up, they're generally quite equal. However, if enough time and money are put into a PS3 game, it can surpass the 360 (such as KZ2).
I'll try to put this into another point of view. When you develop a multi-platform game, you have to take in mind that all features must be capable on both consoles. This means that the power of the PS3 can't really be pushed by multi-platorm games (this is also true for the 360, but to a lesser extent). A notable game that pushes a PS3 exclusive feature is Wipeout HD, which lowers the resolution during the game to reduce framerate drops. The difference is then made up for by the PS3's horizontal scaler. This prevents frame-rate drops, while still keeping the game looking beatiful.
That was my two-cents.
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