USB versus FireWire
Debates about USB versus FireWire more or less following the same pattern as Mac versus Windows, you are in for a flare.
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 rate.
If you look at the Firewire DACs, you see a very limited list. Not only because I don’t know them all but because there is no audio standard for Firewire.
This forces a manufacturer to develop a device driver at the PC side.
A complicated and costly affair. Only companies with sufficient budget and skills and customers in need of this solution, can afford this. That is why the Firewire DAC is often an AD converter too and targeting the Pro-market.
In case of USB you don’t have this problem.
There is a standard and all major OS support this standard.
Even at the DAC side you don’t have to do much. A cheap drop-in chip and you have a USB DAC.
Is Firewire inherently better than USB for a DAC interface?
Firewire and USB are just specifications (or several versions thereof). They are just pieces of paper. As such, your question has no meaning. What you can ask is how {device X with FireWire interface connecting to cable Y to device Z with FireWire interface} compares with {device A with USB interface connecting to cable B to device C with USB interface}.
Unfortunately, there are so many X, Y, Zs and A, B, Cs that you won't be likely to get a useful answer, but there will certainly be lots of different opinions.
Epilogue
When I wrote this in 2010, Firewire was the standard in pro-audio.
Over time typical pro-audio companies like Lynx and RME added USB support.
Today even Apple, the “owner” of Firewire dropped support for this bus in favor of Thunderbold.
DACs with a Firewire interface are becoming rare, likewise AD converters.
USB rules in the pro-audio world.
In Hifi the trend is towards streaming.
Some companies even dropped USB DACs in favor of streaming solutions.