USB vs SPDIF, which sounds better?

Only way to know is to try it. Noise is a complicated thing.

In theory, without noise, USB allows your DAC's clock to take over with the least effort.

Sometimes optical is best thanks to zero ground loop issues.

I agree with Eric_squires.   I would try them both as well as the AES/EBU if you can.

I have an Aurender N10 feeding a Chord Dave and personally prefer the SPDIF connection over the USB.  I found it to have a sweeter highs with a larger sound stage.   I’m sure it is very system dependent though.

Thanks for your input.  Yes, I will definitely give the SPDIF cable a try.  From what I've read/heard so far, it is likely that the digital SPDIF setup will sound better than the USB setup.  The question now is what digital cable to get.  I've read that Snake River Boomslang and Purist Audio Neptune are really good cables.