A lot of it is a hold over from generations past. The current (about to be last) gen is the first one with such a small gap in power between the systems. Before this generation, the consoles had always been massively different in terms of both power and architecture, which meant that two games with the same title were often two completely different games, because everything had to be re-built from the ground up to work on the other system. It's why exclusives used to mean something -- developers did it more because they couldn't afford to port to more than one system, or because technical limitations made it impossible, than because of some agreement with the system's manufacturer. The current systems, however, are pretty much on the same level in terms of raw power. The difference is in the way you have to optimize for the architecture. The 360 is pretty much just a PC under the hood, which is why the 360 tends to have the better multiplatform ports of the two main consoles, with the PC winding up with the best one overall: more people have either a 360 or a PC than have a PS3, so it's a safer target. The PC beats the 360 because the 360 is almost ten years old, while PCs never stopped getting more powerful, so games just run better on that more powerful hardware. Meanwhile, the PS3 tends to get worse ports not because it's less powerful, but because the Cell processor is really difficult to program for, and to do it properly would require re-programming the engine from the ground up, which most companies don't bother to do.
Even then, though, the worst you see in most PS3 games is the occasional bit of screen tear where it loses V-sync. The exceptions tend to be huge RPGs (typically made by Bethesda) with massive amounts of stuff going on in the background, and that's because the PS3 has a really weird memory setup, with half of it only going to the GPU, and the other half only going to the CPU. The 360 has the same amount of RAM, but all of it can be used for whatever is needed.
The PS3 also often has better textures than the 360, because Blurays have more room on the disc for hi res textures than the DVDs that the 360 uses.