Good question. Theoretically, an Asynchronous USB connection would have lower jitter due to the way timing is controlled.
There's also the issue of EMI/RFI from the various signals bouncing around inside the PC, which of course don't apply to the external STU.
Another thing is the power supply - the STU has an external power brick kind of like a laptop would have. It probably falls somewhere between a basic wall-wart and a dedicated linear power supply. Is it better than the PSU in your computer? Depends. Some people have really nice PSUs in their machines, others don't.
Lastly, and this might be the biggest deal - volume control. When using the ST/STX, all volume control is handled in the digital domain. That results in a loss of resolution. Is it audible? Probably not, if you only reduce it to 90% or something. But who listens at nearly full blast? Most listening is likely to be done in the 20-50% range, which is almost certain to cause audible degradation. The STU doesn't have this issue, instead relying on a pretty good quality analog potentiometer to adjust volume. Note that when using an external headphone amp, this difference no longer applies, since the ST/STX could then be used at 100% volume, and the external amp would be used for volume control.
Thanks for that information Project86.
I currently have the STX and I don't hear any interference within my PC. I also have a Corsair HX850 watt PSU in my system. Are you saying the volume in Windows should be at least at 90% to avoid any loss of resolution with the sound card?
Anyways, I'll probably hold off on an external DAC until I see some direct comparisons between the STU and STX.