No preference it depends on the equipment and cable. That being said BNC is basically a superior version of spdif so should be better than RCA.
Cable for Transport to Dac
Have a Jays Transport on the way for my Bryston Bda-3
Have the option of 110hom Xlr 75 ohm Bnc and 75 ohm rca
Is there a preferred type or what sounds best in the system