OK, so what differences would someone expect to hear on an optical versus a coax connection if the optical were worse (or even different)?
I have heard some pretty bad optical connections in cheap equipment, but I can't say how much was due to bad optical or simply bad electronics period. These are pretty gnarly and obvious under par.
However, I am not convinced that I can hear a difference between optical and coax on a decent or better piece of equipment. There are those that state they can - which may very well be true. The differences in jitter can be measured however.