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

This construct of "which do you prefer, XLR or RCA cables" is kind of backwards. Choice of XLR vs RCA (single ended) cables follows the design of source equipment such as DACs & preamps, in which either balanced or single ended designs may be executed for reasons best known to the designer.

true - but there are equipment/system choices that can be made using one method or another ...  thus the question is relevant to many

i agree that for some, long runs across areas with strong sources of interference will show the benefits of xlr connections and cable, but i suspect that in just as many (if not more) cases the apparent sonic ’benefit’ is due to the balanced source delivering twice the voltage swing to the receiving component

lots of people fail to properly compensate for volume differences and are swayed by the subtly (or not so subtle) louder version they hear

in any a-b comparison loudness compensation is absolutely critical -- our ears are impressed by 'louder', even by fractions of a decibel

all else being equal - xlr

but the very nature of xlr is NOT equal to rca... at least if xlr is properly implemented

hope op understands that

a balanced circuit is not always behind a balanced connector on a backplate

this being said, single ended circuits and connections usually work just fine