Optical or coax?

If there is a choice of using an optical or coax IC, what is the best to use? Are there any advantages of one over the other?
Are you talking about real glass fiber optical (AT&T ST) or plastic* optical (Toslink)?

* Toslink is available now made w/ glass fiber, but still has the same limitations as the plastic stuff.
My understanding is that if you are coming from a transport that has video capabilities toslink works better. I've tried both in several situations and can't hear any difference. But then, my ears are getting old. (My mind remains firmly planted at about 17 years).
I would be using a "quality" glass fiber that could be used with the optical output of the mc 861.
OK, if you are referring to the Mcintosh MC861 DVD player, I would definitely recommend using a high quality digital coax. Its "optical" output is Toslink, and although you can get the ICs in glass fiber (as opposed to the standard plastic fiber) it's the transceivers (the thing they plug into on each end) that limit their performance. Also the Toslink cables themselves (glass or plastic) aren't good for much over ten feet if I remember correctly (AT&T can go for miles.) Anyway that's my 2 cents.
I use optical to run the CD data to my Krell DACs in the pre and coax to run the digital cable info in to the Krell. Fairly equal sound quality IMO. One thing I was told and which makes intuitive sense to me is that optical doesn't carry electrical charge while the coax does.

Jcc3 -- I believe that w/ your Krell stuff, you're talking about AT&T ST glass fiber optical connectors, and not Toslink. And to clarify the electrical thing, it's not so much about carrying a charge but rather that fiber optic ICs (Toslink OR AT&T) are not conductive and therefore the equipment is electrically isolated from each other -- sometimes an advantage when hum problems arise.

As for the glass (AT&T) vs. coax or AES/EBU issue, it really depends on the quality of the cable in both cases. If you are talking about premium examples of either kind, for instance, Aural Symphonics Optimism 2 AT&T glass fiber ($1000) vs. Purist Dominus or Siltech coax or EBU (>$2500), I would have to say go with the glass (AT&T, not Toslink) if your components have AT&T inputs/outputs.

In a choice between Toslink and coax, I'd say go with the coax if it's reasonably high quality (in the $200+ range.) If it's just junk coax, you probably won't hear any difference between the two.