Firewire has two bandwidth specifications, 400 Mbps and 800 Mbps. It's also known as 1394 A/B or i.Link depending on brand.
USB2 has a bandwidth specification of 480 Mbps. On paper, USB2 is faster, however, in thea real world this isn't completely accurate.
Firewire 400 is mainly hardware based, and its great for steady bandwidth, so it will usually work at high speed. USB2 doesn't work the same way, not only does it tax the processor more than Firewire, but sustained thoroughput is not as great, despite having more bandwidth. USB2 is fine for burst operations, and will likely reach high speeds.
Therefore, it really depends on your application. Hard drives will usually perform around the same on Firewire 400 or USB2, but the processor will definately be used more for USB2 operations.
Firewire 800, or 1394B blows them all away though.