This is where the Blu-Ray disc really starts to shine, The 360 and PS3 may be on level playing field processor-wise. But because of Blu-Ray and its 25gb per layer disc, PS3 has more potential to make the games bigger, and better looking without compromising the game length/graphics.
I believe that we have been seeing this with recent games, 360 developers often lower the game resolution so they can fit the game onto a 4.5gb per layer disc.
For Example, Alan Wake had the potential to make the resolution 720p, but its assumed either they got lazy (unlikely for a game that has been in development for 4 years) or they couldn't find a way to fit the game onto a single 9gb dual layered disc without shortening the game (thankfully they didn't), so they reduced the game's resolution.
For another example, Final Fantasy XIII, which I think was handled better, Square Enix split the game onto 3 dual-layered discs, although the game's resolution was 576p (instead of the 720p on the PS3 version), the game wasn't drastically different form the PS3 version.
I think in the near future most 360 games will be made on more than one disc.
