I will be first to admit I have very little experience between the two, but in theory I would have guessed optical would be superior in bandwidth (capacity), signal quality and immunity from interference. Plus being a smaller cable it would be easier to work with.
In my experience with fiber optic interfaces in the computer industry, fiber optic is generally regarded as having higher bandwidth over copper, and is preferred for most interfaces to high-speed devices. But maybe this concept doesn't necessarily apply to music as it does in computer channels.