AFAIK, usb ports are only for transfering files to and from your computer. Sometimes i use it as a charger. But using it like say, an external HD where you do video or large psd files, go firewire. Well at least thats my opinion.
If you are refering to usb 1.1, it is usually sufficient for 2-channel I/O when it comes to audio. Usb 2, OTOH, has a max data bandwidth that is actually greater than Firewire 400 (so-called 1394a). Max bandwidth can be misleading, however, since what is important for audio is sustained bandwidth speeds. Depending on the pc configuration, USB 2 interfaces CAN compete toe-to-toe with Firewire 400 interfaces; the problem of higher cpu utilization with usb is what puts off some users. Obviously if you have a lower spec processor, usb will demand more from it than a higher spec processor - I have yet to see how dual/multi cores can possibly affect system performance since you can, in theory, allot one processor to the system/software while the other processor controls the usb2 interface. In practice, however, the interface's usb driver coding often does not take this into account.
As for firewire cards, it's usually recommended to use Texas Instrument chipset FW cards. Via firewire chipsets, both card and onboard, have performed inconsistently for audio use. AFAIK, the only interface that works well with Via is RME.
