I believe the communication and connection of the cores in the X360 system and SPE's in CELL are much better than what is supplied to VU0.
Correct me if I'm wrong, but doesn't VU0 have to go through VU1 to access main memory? This would present a bit of a problem since VU1 also has many needs for those resources.
Fafalada, Deano, or ERP will probably eb able to chime in here and clear this up. My apologies to those that I missed..