neither are stupendous on the stock unit, but most feel the coax is best. I felt the Toslink gave a bit mellower tone if you are into that while the coax was a little more detailed.
If you have a few bucks to spend, consider one of the modifications available for it that will put it on par with better CD transports (Cullen Circuits, Emperical Audio Pace Car, etc). I did a Cullen mod on mine and have never had cause to fire up my CD player again.
