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.

OCD HiFi Guy (Mike Powell’s YouTube channel user name) tested USB vs SPDIF cables for their respective SQ. The SPDIF cable was the clear winner, despite having the digital signal go thru a USB-SPDIF converter. The link is below:

https://youtu.be/WT465YxYr-Q

A true SPDIF digital cable is one terminated with BNC connectors. Those in the know (audioengr) would recommend a SPDIF cable having a 4 ft length. RCA connectors aren’t true 75 Ohm connectors. So if your transport and/or DAC only have RCA connectors, then just get a BNC-terminated SPDIF cable and 2 BNC (female) to RCA (male) adaptors.

(From what I can gather via internet searches, your server has a BNC-SPDIF jack. The Big 7 DAC appears to come standard with an RCA-SPDIF jack, though one can outfit it with a BNC-SPDIF jack as an optional, extra input. So you only might need one BNC-RCA adaptor in a “worse” case scenario.) 

Thanks @celander. You are exactly right. My N10 has the BNC output, but it’s my DAC that’s limited to the RCA connection. So, I may end up using one BNC/RCA adapter on the DAC end. In Mike’s video, I don’t see what USB and digital cables he used in the demo. I know different cables would make a difference, so if he wasn’t using the same level of cables in the demo, then it would sort of skew the result. Anyhow, I do believe everything being equal, the SPDIF should sound better than the USB. Now the question is what digital cable I should get. Any suggestions?