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?

I used a number of SPDIF cables. One is a $500 BNC-terminated solid silver cable made by Mark Coles from Sablon Audio for Here’s his link:
http://sablonaudio.com/index.php/signal

Oyaide makes a decent, $150 BNC-terminated SPDIF cable composed of 5N’s silver braid (DB-510, 1.3m).Here’s an eBay link:
https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%2Fulk%2Fitm%2F283160127875

Mike Powell (OCD HiFi Guy) will respond to you quickly regarding what he used for SPDIF cabke(s) if you reply to his YouTube video clip or message him here.

respected_ent,
As a new N10 owner I'm still learning too.
As I understand it N10 USB out is not via the Aurender N10 clock but by your DAC clock. The Aurender clock is amazing and should be used.
DSD only feeds through USB so for best effect we would require USB & SPDIF cables attached at the same time, the N10 supports that. SPDIF for everything except DSD will use USB.
I've only used a modest Oyade USB cable so far but plan to get a SPDIF coaxial very soon. I was reading your topic hoped to find more information on a quality SPDIF cable.

The ground loop issues of a USB cable can be mitigated by using an unpowered version (data only) of the USB cable. Several high end companies offer this sort of cable (Sablon Audio, for example). 

@respected_ent  Mike Powell goes by the user name @mikepowellaudio here on Audiogon. 

@respected_ent Your DAC should be used to select the digital input. 

I just double-checked the back of my Big 7, and there is no toggle switch to switch between USB and SPDIF.  Does this mean I can only have 1 cable physically hooked up at a time?  I may need to check with Lampizator on this.

It’s odd that such a high-end DAC wouldn’t enable user-selected options between digital inputs. I’m sure the company can assist you with this.

My bad.  I forgot I have a remote control for the DAC.  I usually don't use the remote because I have an integrated amp, so I always use the volume control on the integrated amp, not the one on the DAC.  Using the remote, I was able to toggle the input selection.  Problem solved. 

I am a former Big 7 owner ( previously a Big 6 and an original model 4 as well... ) currently running a Golden Gate. Loved each one for their own best virtues and kept climbing the Lampi ladder... so far, so good. Big 7 is a fantastic dac with outstanding value for the money. I ran a few different S/PDIF’s from my cd transport and USB’s from my Lampi Komputer.. 
Top 3 favorites in no particular order are Mad Scientist, Triode Wire Labs, and WyWires. 
My recommendation for excellence and best bang for the buck is Triode Wire Labs... Pete makes outstanding cables at very reasonable prices... he’ll build anything you need the way you want it... has a 30 day return trial policy and is a real pleasure to deal with. 
If you’re looking for a remarkably natural presentation, I can’t speak highly enough about his products. 
PS: which DHT’s and what rectifier are you running? Just curious...
Good luck and happy lissn’n! Lissnr

I've had an Aurender N10 for a year or two and have tried all of its digital outputs.With that player, they're all pretty good. To my ear, here is the stack rank from best to not-so-best:

XLR: quietest with the biggest soundstage, great midrange & bass,  and most organic sounding (what I use all the time)
SPDIF RCA: very, very close to the above
Optical: not a very deep soundstage, kinda "thin" sounding, but surprisingly good for optical
USB: not as good as any of the others, but OK

I have limited experience with brands of cables and currently run all Shunyata Sigma cables in my primary system save for the speaker cables. I'm sure that there are many good cables out there, but I worked up the Shunyata product line, so haven't tried others. 

@medwardo, thanks for your info.  I recently got a Silnote Poseiden Signature SPDIF RCA cable, and so far I like what I am hearing.  The SPDIF RCA connection does sound fuller, bigger soundstage, and more detail than the USB connection.  I also feel more emotionally engaged to the music than the USB connection.  I may try some more digital cables later on to see how good this can get.

Now using a PS Audio Memory Transport into an upgraded Yiigdrasil SPDIF
Using a WyWires Litespd cable. This cable has a special quality to draw you in to listening. Its the best I have heard in  the last 3 years here on different DACs. Was using the AES connection on the PSMT with a balanced Belden 8402. Very good also.
The Belen killed a DHLabs DH110 which is supposed to be the required 110 ohm
Standard. So, the connection quality varies with equipment used.
The Lifatec optical cable comes close. USB is on the bottom of quality.
Even with all the decrapifiers, been there done that too.

Since cables sound different in different systems based on synergy, I would say:
1. If you compare AU24SE USB, only compare to an AU24SE SPDIF, and an AU24SE XLR. 
2. IMO, SPDIF is the most reliable in most situations, out of the 4 (USB, SPDIF, XLR, Tos) from DAC to source. 

3. IMO in general: 
1. XLR or SPDIF are generally the best, followed by USB or Toslink.
* XLRs tend to have a bit greater dynamics from top to bottom, blacker backgrounds. Sometimes perform worse than SPDIF.
* SPDIF, clean with good dynamics and all-around performance. Sometimes perform better than XLR.
*Next best is USB or Toslink, depending on the brand of the Tos and the type/brand of digital sources/DACs. USB can carry more noise. and Tos can be flat or lively and musical. Again, depends on the units it connects. 

ST Fiber: better than people realize, I use one to connect to my Bel Canto DAC. the SQ is smooth but still rather punchy.
**I would contrast the ST Fiber with an XLR: the XLR is more dynamics from top to bottom, more clear sound from instruments-- but this can make the SQ appear to thin b/c instruments and hi/med/low frequencies, in the name of "clarity". I use quotes b/c it is a debatable issue.  

I use BNC over SPDIF when I can:
BNC connections are generally more preferred than the SPDIF connections, since the BNCs lock into place, but the SPDIF RCA connection can come loose. 

On the back of your DAC, you can install a BNC adaptor where your SPDIF input is. Then use the BNC connection every time, locked in. 

Some manufactures offer a combo SPDIF termination: Like TWLs or Nordost Blue Heaven, they are technically BNC, but they are sold with 2 gold plated adaptors for each end, to connect to digital source or DAC. 

I generally like the SQ from the TWL cables I have: SPDIF/BNC, RCA ICs, XLR ICs
Best