Left and right XLR's are analog outputs, not digital outputs.
Single XLR is usually AES/EBU digital output and input.
S/PDIF coax digital output can be even better than AES/EBU. AES has one advantage and that is common mode noise rejection because it is differential signaling. The disadvantage of AES is that the XLR connector is not 110 ohms and good cables are more difficult to find and usually more expensive.
S/PDIF coax digital can also be isolated using a pulse transformer.
Steve N.
Empirical Audio