In my experience with the PS3 version, I've only noticed the framerate hover below 30 fps a couple times. One was when I was playing online on the revolving elevator on 5-3. Still perfectably playable, but there was a fine choppiness to the capsules passing in the background. Another was one time at Public Assembly in 1-1, where the area looked fuzzy when viewed from afar at the top of the tallest watchtower.
Other than that it's been about as smooth as any other PS3 game, including ones built for the system from the ground up. I'd take that over any possibility of tearing. The triple's still perfectly responsive, you still see all the detail, the colors are warmer, the controller's great and the online's free. Sound sampling is consistent, too, but that may be my TV or the hardware. I'm using an 80GB PS3 via HDMI on a Vizio 42-inch 1080p LCD HDTV, with the system outputting in 1080p. It's been great.
I have noticed some slight color differences between the two versions. Nothing major, but the 360 tends to have more graytones while the PS3 version looks a bit warmer and saturated with light. One place I noticed it was in the cutscene before Public Assembly, on the Agitator's face; on the PS3 version, the sunlight shining on his face is stronger, which I liked because it made the scene feel hotter, while the 360 version was just as good in its own way, because on the 360 there's less sunlight but you see more detail on the Agitator's face, detail that's there on either version but more visible with less light. Both versions looks beautiful.
I haven't had a chance to play this game the last couple days, but this weekend... This weekend, I'm hoping my splitscreen partner and I can put to use all of the guns I fully upgraded for the two of us. I want to start a Pro run!