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

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.