USB vs SPDIF, which sounds better?

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

Regarding cables the best cable I have even heard is High Fidelity Audio spdif  magnetics technologies the Ct-2 if you call direct 
around $1k and beat 3- usb cables 3x it’s price in realism and detail.

I think it really depends on your source and DAC.  I have an Auralic Aries G2 server going into the DAC built into my NAD M12 DAC/Preamp.  It’s a decent DAC - not great - but pretty decent.  There’s reason to believe that newer high DACs are moving in the direction of USB and Ethernet being the preferred connection.  

I had my server hooked up via a Nordost Heimdahl digital cable (1.5M).  It was superior to the Audioquest digital cable I previously had (don’t remember model but it was probably $300 when I bought it).  However, based on another thread, I thought I should try the USB connection and bought a Nordost USB cable (much less than the Nordost SPDIF/Coax cable).  And, it’s a bit better than the SPDIF connection.  Not loads better, but a bit more depth and openness.  Now I may try the Nordost Heimdahl USB cable.  (Where does it end...).

I know an Aurender dealer who prefers the USB connection on his N10 going into his DAC.  YMMV.  

So, I think the only way to know is to audition it yourself. It seems to me to be very system dependent.

Best,

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).

Most standalone DAC's don't take power from the USB cable anyway. Power isn't the real culprit, the ground loop can occur in the data, not to mention extra noise. Ideally DAC's now have galvanic isolation built into all of the metal connectors.

If your DAC does take power from the USB cable, using an isolator with a linear power supply is a better solution.