I have the Cynosure by Locus Audio. It was crazy expensive and I haven't compared this cable for years as its been the only one I have used for a long time. The recent use of dual terminated usb cables completely separating the power and signal conduits has piqued my interests. The cynosure was one of the first to give credence to that theory, but carries the separated, insulated conduits in the same cable. I can say it it sounds amazing...but also that I am moving on!
So, I looked at the Kingrex, the Ridge Street and a number of others with dual origin terminations and separate cables....although rejoined at the end just before the signal enters the USB DAC. I think a battery supplied power leg is probably best, but no way I would keep up with recharging it. Since I use my AMR CD 77.1 as my USB DAC, I went with the iFi 4 part system. They are very much scientists in pursuit of great sound and signal paths that convey that, so it makes good sense to me, matches my equipment (a tidy little sonic ecosystem) and I really like their sound. The player and DAC are the best to my ear I have ever heard.
SO...MacMini (DC modded with Mojo Audio)-->1m Mercury USB (a single iFi cable)-->iFi iUSB-- (a dual iFi cable)-->iPurifier--->AMR is now my signal path. A bit quieter and very nice sounding, too! And, about 550usd total.
All that said, I do think there are differences in USB cables and they aren't subtle, but take system matching and synergy like everyhting else. My buddies has MAC 6700/Esoteric/Music Vault/Cremona M and uses the Crystal Clear Audio Master Class usb for sale here on Agon to amazing effect...
As for a digital cable...sp/dif? Pick up a Stealth Varidig and you'll never look back. Unless you want to pay 3x that for the Sextet. IMHO.
Good luck, enjoy the search and always enjoy the tunes!