I have both and use both. For sustained, heavy data flow, such as burning a DVD, I use Firewire. For quick small jobs, USB 2. There are more USB devices around - that's the mass market. Firewire is a more mature technology - derived from SCSI III by Apple - hence, there is a bias against it from Redmond, WA. Firewire B leaves USB in the dust but there are few devices that use it.
I use Firewire for external HDDs as well. USB 2 for media readers, etc.