Asynchronous USB delivers just data - you cannot make it any better. The only disadvantage in comparison to S/Pdif is the amount of electrical noise injected into DAC, that can affect the sound. When you have both cables connected USB should win.
BNC and USB from streamer to DAC
So, i’m sure to some this will be a stupid question but: Can i connect both USB and BNC from my streamer to my dac, so i can compare sound, and just switch sources on my dac between the two?