SQ difference XLR vs RCA ?

RCA and XLR connections can and do sound differently.   I have a Gustard X26 Pro DAC that supports simultaneous RCA and XLR outputs.  I connect them using Harmonic Tech Pro Silway RCA and XLR cables to a PS Audio BHK Preamp which drives a 2A3 tube amp and Tekton Moab speakers.  It is easy to switch inputs to compare the sound of XLR and RCA.  There is a 6 dB difference but that is easy to correct using the volume control of the BHK.  The XLR is brighter and more detailed sounding which is fabulous on classical and many sorts of music but can be a tad fatiguing compared to RCA on tracks that might sound brittle or shrill with that brighter more detailed presentation.  Now I haven't figured out if the difference between XLR and RCA is due to the DAC, the Preamp, or the cables but the fact that it exists is indisputable and would be easily heard by anyone that knows how to listen.  Frankly, I like the fact that I can alter the sonic character so easily by changing the input.

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.