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

Either.

That's why 'gender-benders' and whatwherewithall exists for your purchasing pleasure.

'That' is all rca....whereas, 'this' is either xlr or 1/4" phone.

Except for the f/o, that's pretty straightforward....

One adapts....*s*

Hello, I am running a Parasound P6 Pre with Parasound A21 amp feeding it with Yamaha cd-s1000 sacd player, and  a musichall 7.3 with Ortofon Black 2m  for my turntable, all feeding my B&W  803s's.
I am running Audioquest Water RCA's, and XLR's. I did notice the different between the RCA and XLR. The XLR's  have a more open sound stage compared to the RCA's . Imaging was better. But i only can use the XLR between the Pre and Amp, every thing else is RCA's.
My other system Mcintosh c42 pre and dual mc162 amps. I really noticed the difference between the XLR to the RCA's.But it also depends on the cable, not all cables are made the same or sound the same.

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.