I'm going to have to partially disagree with gmonkey. I 've used quite a few USB2 and firewire devices, and I notice particular differences between them. In my experience USB2 is slightly less responsive in some circumstances.
Specifically: slower in initially accessing a device or accessing a device after a long period of time. This manifests as a slower "wake-up" time to populate file/directory lists in Explorer, and slower initial response upon file transfer (read or write). USB2 also seems to "cheat" more often than firewire when I perform moderate to large file writes to the external device - on the surface it appears to have transferred files, but has often only set up a delayed write transaction. That shouldn't be a problem for most file operations, but could be problematic if a bunch of large writes start getting pushed to the device, ala batch image/audio/video processing. Firewire has to do some queuing in the same circumstances, but seems to handle it with a lower frequency of errors. Simultaneous read and write from/to external devices also appears to be better handled by firewire.
All of that is anecdotal, however. It's been my experience on about 6 different machines with different FW and USB2 chips (primarily TI, VIA and Oxford Semi) and varying devices - mainly external hard drives, peerless drives, and CD/DVD burners. As always, YMMV.
Edit: another reason I'm partial to firewire is chaining - the ability to make the firewire connection from one device to another instead of all devices needing an equivalent plug on the computer. This is great for adding more than one external HD - just stack them and connect them.