usb dac for hi-end system

"On paper", I'd stay away from any S/PDIF given that S/PDIF require one to derive the clock from the input signal. Asynchronous USB DACs don't have to do that and jitter is extremely low - not induced by the data link mechanism. However, in the end, jitter and how the data is transported doesn't make any difference if you like what you hear. Also, for any computer based front-end, sending PCM out USB is the way to go.

Why does it matter if it is in packets or not? Some S/PDIF implementations buffer on the target, so why does it matter how the data makes it to the DAC? Also I don't see where Firewire data is not "packeted". All serial based links (serdes) use data packets. Firewire has some extra features that allow higher throughput than USB (less host interaction) but for single stream PCM it shouldn't matter. Unless you show some real data, I'm raising the BS flag.

And to add what to Jeff said, it doesn't make a difference if you use firewire or USB because all you are doing is sending bits from one place to another. The better implementations using either interface do not try to recover clocking from the input data stream. So, in this case if the sound is really different (vs imagined) and all other things being equal, the bits would have to be changed to account for the difference in sound. This is easy to prove right or wrong. Just copy files between a USB drive and something else with data integrity checks. How many times will the target data not match the source data?

And, most likely many "professional" studios/people use firewire because it was the interface of choice on MACs where just about all the "good" music software started and still exists. And it does have some advantages over USB, especially for multiple devices which not the setup most people use to take PCM from a disk drive and send it to a DAC.