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.

I have 2 balanced headphone amps in my system (which also sport SE outputs); and 4-5 single ended headphone amps; and a big speaker amp that can be run via SE or balanced cables. So I've had a lot of chances to compare SE vs balanced, across amps. Here's what I've observed:

-- Running an all balanced system (source components & preamp & speaker or headphone amps all designed for balanced operation) might be ideal theoretically. But due to the greater output voltage coming from balanced vs SE source components (2.0V - ~3.0V SE vs 4.0V - ~5.0V balanced), a balanced chain would introduce intolerable gain issues that are not possible to eradicate without introducing ancillary gear (ie, passive volume controller) in one or more places to control gain (and that's hardly ideal).

-- Headphone amps designed for balanced operation usually sound better via balanced output cable to the headphone vs single-ended cables. Headphones are quite revealing and these differences, though subtle, are not a big challenge to hear. Balanced cables often deliver slightly better/wider/deeper soundstage, dynamics, and bass.

Having said all that--if I ran a music studio and my audio gear was in service of music production, everything would be balanced--because long runs of balanced cabling are far more immune to RFI and other external distortions than single ended cabling.

By far, the biggest difference between unbalanced RCA and balanced is on long runs of cable.  Short runs of unbalanced work very well.  I use 18-36 inch connects, only.  I have A/B's these on my system. At about 20 feet, I can find a difference if I get totally anal about it. 

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

If you can get your DAC or preamp near the amp there is no benefit from a balanced connection, conversely if you want to drive mononlocks near the speakers balanced is likely the preferred option. In high res systems with short cables my experience is that balanced depending on implementation at worst is more sluggish or in the best of all worlds gives no discernible benenfit. Longer runs, balanced wins.

Xlr vs rca.  No one can tell the difference when runs are shorter.  AB testing impossible due to volumn level changes.

if i had a choice i would go xlr, cause its alot cooler to say i use xlrs.

so go for it