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

To the simple question @kingbr posed I prefer XLR.

On a side note, this thread illustrates the best and worst of forums. While I truly respect (and I believe most do) @millercarbon and others who are far more knowledgeable, decorum might suggest another way to join in / reply. And yes, I may be inviting a host of sarcasm and hate. 

I've yet to meet anyone who has audio-omniscience. One reason I enjoy Hans Beekhuyzen is that while he occasionally employs mild sarcasm, he tries to educate and explain. I think we all (?) enjoy learning, reading, listening, exploring, and enjoying different aspects of audio. I continue to learn from experts and a lot of personal experience buying, A-B testing, "upgrading", etc. To me, this is all part of the enjoyment.

I've got a very nice headphone system: dac connected to 2 different amps. I tested (A-B) with RCA and XLR. I'm no millercarbon but I could tell a significant improvement (BTW using 2 different balanced openback headphones and 1 IEM, all with balanced cables) with XLR over RCA and the cost wasn't that much steeper than good RCA cables. Additionally, my main stereo is all XLR balanced and I can also tell a difference.

As Hans closes his YouTubes videos, "...and whatever you do, enjoy the music". :-) 

While not about RCA v XLR, Han's latest is a perfect e.g. of decorum, imo. 
https://www.youtube.com/watch?v=HqBYB6kEKec 

In most posts, people are describing their preference between a dual differential balanced circuit and a single end circuit. If you compare a xlr cable with pin 1 and 3 tied together with a rca cable of the same make in a single ended circuit, they should sound identical. 

If you have differentially balanced equipment.....balanced cable is better...with no problems in the association.   I have Dif. balanced CD player, amp, preamp, even the cartridge is wired balanced. (XLR phono)   No problem, only music.

Assuming your equipment is taking advantage of the balanced connections, then this is the clear winner in my book.

RCA cables carry the signal on the same connection as the ground currents between devices which is a bad idea. If all your gear is well design and plugged into the same distribution block with good grounds, RCA cables can work ok. But well designed balanced connections which isolate the ground currents from the audio signal will always be at least as good and usually better. 

Balanced connections also provide much better common mode noise rejection, so it there is any RF or EM noise generated by your components or other devices in the surrounding area, balanced connections provide a much cleaner signal. 

At this point in my audiophile journey (45 years in), I would not consider products that didn't support balanced connections.