USB versus FireWire

Debates about USB  versus FireWire more or less following the same pattern as Mac versus Windows, you are in for a nice flare.
A good discussion can be found here: http://www.diyhifi.org/forums/viewtopic.php?f=2&t=1650
Observe that some of the participant are manufacturers of USB-DAC's.

A good treaty on FireWire as a bus for audio by Julian Dunn.

 

Being the first, Firewire had an advantage over USB but USB improved strongly over time.
I think today sound quality is not depended on the bus chosen but depends on the way it is implemented

• .Both are capable of bit perfect transmission
• Both can induce input jitter
• Both can be used in adaptive and asynchronous mode.

Asynchronous mode is considered best for audio.

 

Windows and OSX have native mode USB audio drivers.
In case of  Firewire one has to write one’s own audio driver. This adds to the cost

 

USB audio using native mode audio drivers support up to 24/96,

The new USB audio standard (version 2) also supports 176 and 192 kHz sample rates but is not implemented as native mode drivers.

That's why sample rates > 96 kHz needs a third party driver.

 

If you look at the USB-DAC page it is rather limited, (not only because I don't know every single USB-DAC). Note that big names, companies who has a reputation in high end audio are not in this list or if they are, their offerings are very recent.
There is still a big gap between traditional audio and PC audio.

In case of Firewire, the offers are even more limited.