SQ difference XLR vs RCA ?

Another factor, some equipment's XLR inputs/outputs are not really truly balanced. The XLR connections were just added for convenience. Unless absolutely necessary I would never use those and would go RCA in that circumstance. This is a question you would need to ask the manufacturer when in doubt.

I use a short .5m XLR connection from a Macintosh SACD (MCD85) to a Macintosh MA352 IA.  I was originally using the RCA outputs and inputs for SACD/CD.  It was a noticeable improvement in sound quality on many SACD & CDs (not all).

My conclusion is that the XLRs brought out the most in the medium and equipment depending on the quality of medium. 

Just to be an old curmudgeon, RCAs have worked just fine for me over the last 55 years.  

Balanced cables always sound better whether you can hear it or not.

Balance components are basically designed to have one side mirror the other side with respect to the electronics inside the chassis.  When manufactures do this they tend a cheap out because they have to do two systems.  So if you get a RCA only device there's no mirroring of the internals so the belief is that more money spent on the electronics you're buying.  Conrad Johnson is a perfect example, they only do RCA connections.