The architecture is radically different.  USB peak is slightly higher. FireWire sustained is significantly higher for hard disks.

DVCAM throughput is the same for both USB2 and FireWire.

Multiple devices on the same USB tree do interfere with each other. Firewire, on the other hand, is direct from source to capture program with no intermediary processing or USB2 type limitations on throughput.. Thus, firewire produces more consistent results, with less chance of frame dropping.

The above information was summarized and based on info posted on the Avid boards  - thanks to Mike Shaw and  Diva.