RCA or XLR?

@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.

The only reason balanced line is needed is in long cable runs.

It’s all about design and synergy.  As already noted, if your great is fully balanced, it generally sounds better with XLR cables.  If it’s designed for single ended connections, will generally sound better using those.  Some gear has XKR’s but isn’t fully balanced.  It’s really about the gear and its design.  I have fully balanced gear currently, from my DAC to my pre to my amp.  They all sound better using the XLR’s due to design.