The 360's GPU has a small edge in performance over the PS3's, as I understand it, but this doesn't usually show through in games, nor does the different memory architecture, as 360 apps tend to either split the 360's memory down the middle, making the architecture effectively the same as the PS3, or they use more memory for the CPU/app, and less for graphics.
The only real potential (note that this rarely means "realized", except for occasional exclusive titles) difference between the two platforms is this: The SPUs can act as "GPU helpers", and very powerful ones at that. In addition, there's a CPU side of graphics that the... uninformed.. tend to discount. Namely, animation performance -- blending animations is a colossal amount of work for a processor to do, and this is exactly the kind of thing the Cell excels at. It doesn't matter how many polygons you can push to the screen, if your animation blending was too slow to handle a large blend stack, or you only have a few animated characters onscreen at once. The PS3 blows the socks off the 360 in this dept -- but you'll never see cross-platform games perform differently due to this fact (except *maybe* team sports titles, eventually...), because the game needs to be designed fundamentally different to take advantage of it. As an example, you design your game to have 10 characters onscreeen at once... or 50. One way shows the power of the Cell, and causes you to lose a heckuva lot of money on your 360 port, and the other way makes both versions less impressive, but allows you to design the game to focus on the less impressive design.
You'll only ever see the difference in the exclusives, and even then not a signifigant number of them. Platform focus is why 1st/2nd party exclusives (like Uncharted, Heavenly Sword, MGS4, etc.) look so good compared to cross-platform games. As a general rule, the PS3's platform exclusives will tend to outdo the 360s, because it is, indeed, more capable -- it has much more disc space, and your game design can focus on things that the 360 cannot handle.
Games like Heavenly Sword used very early versions of PS3 middleware, I would wager, but you can still see a bazillion characters onscreen, animating in the very first scene of the game. The 360 would have to make due with billboarded characters, or by instancing rendered meshes, post-skinning (a la Dynasty Warriors -- the characters would number greatly, but they'd all be in the same animation state, or small subset of states). On the PS3, every guy can be animating in his own way, thanks to the Cell.
In my opinion, most users aren't going to notice such a thing, unless the game design somehow really takes this advantage in stride. Not many games do, or probably ever will. If they do, they'll be pretty impressive, though.