I spoke with the founder of Fractal, whose name eludes me at the moment, at the LA Amp show. In brief he said he uses AES, which he claimed was the pro version of S/PDIF. That said, the guys at Sweetwater who seem very knowledgable swear that when using short cables, like hooking the Axe II to my Apollo Quad the difference is indistinguishable. Further, unless you have a rare device that can take a USB input...as the Mac can't do aggregate inputs...I spent a week fucking with that nightmare. So unless you are recording a guitar track alone, you can't use the USB. That all said, the S/PDIF sounds great to me and I have pretty monster ears for tone.