USB vs SPDIF, which sounds better?

@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