Optical is a very poor digital interface because the LED diodes just cannot light up fast enough to make a pure square waveform pulse. You can see the results here, if you scroll way down to the section that says "What if there is no SP/DIF output (digital out) in our CD player, only Toslink fiber optic type ?"
http://lampizator.eu/LAMPIZATOR/TRANSPORT/CD_transport_DIY.html
Digital COAX is always superior here. Optical should only be used if you have no other alternative. But if you must use it, try to get one of the high density glass fiber optical cables, such as Lifatec or the Wire World Supernova cable.
In my experience, fiber optic gives a very slow messy sound. Detail is just not there.