The best digital audio out of coaxial, or optical?

Toslink is the least recommended connection electrically & sonically. It will "mud" (blur) image and separation. Blacks (background) will become gray and it will soften the bass. Do not recommended it......try to avoid it.
01011001011101001101 (digital interface) is always the same, and it should make no difference in the perfect World - but it is the jitter (Timing, wrong time - wrong note) that screws up the sound (usually during conversion process - digital to analog).

search the net.

You will get answers like that above who claim toslink sucks to it is the best thing since sliced bread. Sorry, you will never get an answer here. Try it and see what you prefer.

TosLink is a low cost optical interface promoted by mass market audio manufacturers as an alternative to coaxial connection. TosLink is trademarked by Toshiba corp. and it is more properly name "EIAJ" (Electronics Industries Association of Japan). Best thing since sliced bread ?????
WOW !!!!! Enough said.

Tubes or SS? Digital or Analog? Silver or Copper? Optical or Coaxial? Toslink or AT&T optical?

As with all things in audio, there is no one right answer for all people, just a myriad of different opinions. You could listen to both, form your own opinion, then give your "professional" advice to your customers......just a thought.

Cheers,
John

Avoid plastic Toslink, glass based Toslink is far preferrable. Coax is good as is AES/EBU, but only if both are properly impedance matched.