RCA or XLR?

I think it matters whether the components are allowing the balanced signal all the way through the system. If so, then XLR will usually sound better. And XLR will provide higher SPL. For instance  on my Denafrips Pontus 15th DAC, I note the following:

• XLR output: ~4.4 Vrms
• RCA output: ~2.2 Vrms
• That’s equates to a 6 dB difference 

If the equipment has an XLR option I will use it instead of RCA plugs.  Not a big difference in sound in my experience but the connections are solid and the cables look cool which adds to the audio jewelry factor.  

@v-fi said:

However some designs actually sound better with one or the other connection type. Even if both are technically available on the components. So you really need to play around with your system and see if you have a preference for the sound either way. If they are equal I'd go XLR.

Exactly.  XLR connections are robust.  But my Audionet Amps and Preamp specify single ended (RCA) cables as best.  Using them avoids the need for an OP amp to convert the XLR back to single ended, as they are not fully balanced.  Joe Kubala showed me how to adjust (tighten) the RCA connectors if they get loose on his ables...easy.

@sealegs read, "pricey XLR's you buy from Guitar Center or some other pro-audio retailer." 

Setting aside the dispute over which sounds better, I fully agree with @v-fi’s observation about the wear and tear caused by unplugging RCA cables each time.