One of the chief elements of appeal for any Bioware game is theme/content continuity over a series, or indeed over separate games. There was a great sense of glee for me when certain staggeringly minute actions performed by my avatar in Mass Effect were shown to have left an impression in the sequel... The third installment would be immensely gratifying for my growing character who would then have experienced an entire narrative arc, while making choices that helped shape his (paragon-slanted) persona.
Then again... I only have part-time access to a 360 and still stand by the superior performance of the PS3 as an entertainment device... So for me, uncompressed data, higher resolution, and a few novelty tidbits sound like wonderful conventions for Mass Effect 3.
Of course, it is more than likely that the game will be clocked to run on 360 hardware, so the cross-console release will mean that Sony's horse won't be racing at full tilt, making graphical distinctions between versions quite minute.