It is slower. Firewire 400 is much faster than USB 2.0 when it comes to transferring data back and forth.
I have a firewire/usb HD and firewire is much better. When I did transfer tests USB was 16.4 MB/s. Firewire (400) was 29.5 MB/s!!!! NICE!!! I would like to see what a FW800 drive could do! When you have a Mac with built in firewire, why take up a USB port when you have a free firewire port? That is assuming you are like a lot of people and don't have a lot of firewire devices.
The Stig