Clovis, there's no technical reason for either the coaxial cable(not "RCA", that's just the plugs on the ends)or fiber optic cable to be inherently superior. Often, as casey mentioned, one output is stronger and/or input is more sensitive, so that one of the setups is louder at the same setting, giving the impression that it's better, but that's just a correctable volume difference. If the respective outputs, cables and inputs are undamaged and working as designed, the transfer is done equally well.
