Unified ram is flexible, but it's never the best performing solution. Both XB360 and PS3 suffer anyway from a quite low total system+graphics RAM of 512MB.
Theoretical XB360 flexibility clashes with the fact that most mid-range GPU's, just like those in PS3 and XB360, use from 256 to 512 MB graphics RAM.
RAM is the biggest shortcoming of current generation, whichever the system.