The only problem is that different implementations of the various receivers can result in USB out-performing SPDIF on some DACs if they emphasised USB circuitry over SPDIF circuitry.
As Mike has hinted above, most problems associated with USB seem to be noise related. Eliminating USB noise is an uphill battle, not least because USB galvanic isolation can be
ludicrously expensive. And USB cables themselves may always generate analog noise when passing on data; this is what would explain that performance may improve when cleaning up the connectors. Last, USB transceivers are generic devices: they're for printing, copying data and ordering pizza --- you can think of them as a miniature SoC, and as such are themselves prolific noise-generators, whichever the specific implementation. (Recall that DACs got externalized partly to isolate them from the noise present inside computers.)
SPDIF transports, on the other hand, are (somewhat) primitive and are designed for one thing and one thing only: streaming audio bits. For Coax, galvanic isolation is relatively cheap and part of the spec (not that all manufacturers follow it to the letter). Whereas for Optical, electric noise is a non-issue to start with. Some will point out that optical can induce performance degrading jitter, and while this was indeed a reality in the tech's beginning, it seems less of an issue nowadays especially if you use a quality glass cable like the Lifatec (which way
exceeds the speeds needed for high-res audio).
So all things being equal, you're more likely to be confronted with implementation issues in USB (some which can hardly be overcome at all) than with SPDIF, especially if you're considering ~$1K devices.
Then there is the pro AES/EBU protocol (which SPDIF is based upon), and reports seem to be consistent that it performs sonically better than the above three. If AES/EBU is an option, it's definitely worth a try, though streamers with AES/EBU like Auralic Aries or Rednet 3 are unnecessarily expensive, so kind of an overkill for something like a Gungnir MB...
I guess, in the end, I'll let my ears decide.
Indeed. Many things may depend on the actual setup, cables, etc.