Either one is a more than adequate choice for the simple application of carrying the digital audio bitstream. Many opt for coaxial because the connection is more solid and the cost is often a little cheaper than the equivalent optical cable.
orangeman
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Being that I am employed with a premium audio and video cable company, Cobalt Cable, I hope that I might be able to give you an educated answer to your question.
This topic is heavily debated, with there being good points for each cable possibly having the best performance. On one had, coax is a good choice because of its durability and ease of use. On the other hand, optical may be a good choice because the cable itself, by not having a metal shield, cannot suffer from possible ground or "hum" problems, which can stem from electrical abnormalities with the electronic components (in my experience, though, these problems are fairly rare, and being that the signal is digital, the point may be moot). Also, and much more simply, there is just something cool about fiber optic cables and connections.
However, my more realistic opinion is that the difference between the two is negligible. I put a higher premium on ease of use, than on absolute performance. For me, I like the increased strength of a durability of a coax cable. Also, I find the coax connection much easier to make behind my component rack, which is usually dark and the connections are usually cumbersome.
I hope that helped. Please feel free to e-mail me privately if you would like any additional opinions or advice.
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You are lucky to have the flexibility...either one will yield excellent results. My Denon 3300 has 3 optical and 1 coax input. My new digital cable box (Scientific Atlanta) only has a coax output therefore my DVD player only can use an optical input...no problems!!!