You probably know this, but the USB does not support sampling rates of 96khz. So there's at least one difference between the two.
I ran the USB for about a year and a half and then switched to the COAX (and shortly thereafter to the AES) because the USB on my Vaio was flaky... too much pressure put on the port by the cable caused the connection to wear and become less reliable.
I never did any bit accuracy tests between the two, but I did need to have different buffering settings using the USB vs. a VxPocket/Coax. With the USB I had to increase buffering settings in WaveLab4 to prevent the occasional pops... with the VX the WaveLab4 defaults were adequate.
Not much help as far as your original question, but I thought I'd chime in here anyway with these subtle diffs.