All things being equal (hypothetical) which do you prefer, XLR or RCA?

If you have a preamp that takes both balanced and single ended devices and a source that supplies both switch back and forth between say three foot balanced cables and three foot single ended ones correcting for any volume differences. See if you can really tell the difference. I just did this experiment with digital cables comparing AES, SPDIF and Optical types. No difference at all over short distances. 

@jjss49 good point about the loudness perception. I actually fell for this way back in the day comparing Definitive Technology speakers vs PSB's. The Definitive's being bipolar (and more efficient at the time) seemed to completely overwhelm the PSB's. However I soon realized after I brought the Definitive's home that they weren't quite as detailed and did not have that razor sharp image that I was looking for for music, whereas as it turned out the PSB's did. The DT's were great for HT but not as great for being able to close your eyes and actually see the band playing in front of you while spinning a cd...I didn't keep the DT's for very long and never looked back...

I prefer true differential balanced, XLR. See the link below, for insight on true balanced cables.
                                               
https://www.ranecommercial.com/kb_article.php?article=2107



   

If all things being equal, I would use XLRs because with my components the XLRs delivery a higher db music signal.

Therefore, I got a bit more volume out of my system.  And as a former field artillery guy louder is sometimes necessary.