My Integrated amp has both optical and coaxial ports as does the digi+. Is the use of one port necessarily better than the other? Which one would you use if you wanted optimal sound quality?
There is quite some information on this question available on this forum.
https://volumio.org/forum/flac-wav-mp3-pdif-out-format-t4110.html
https://volumio.org/forum/need-dac-when-using-spdif-t3898.html
https://volumio.org/forum/advice-wanted-suitable-dac-t4898-10.html
Also helps to search on S/PDIF (the protocol used by these ports)
There are loads of discussions about that. In general I’d say the our category of Pi HiFi enthusiasts will not notice any difference and I doubt whether even hi-end users will actually be able to tell. This is due to the fact that the signal is digital, which is much harder to be influenced than an analog signal. A digital signal simply sends a 0 or 1, lets assume that bad quality cable and interference influence the signal by 20% then the 0,2 and 0,8 that are received will still be processed as 0 and 1. Such an influence on an analogue signal would be disastrous.
Since both signals use the same s/pdif protocol their bandwidth should be the same. There’s 2 minor things you might want to take into account when making this decision.
First, the toslink cables are difficult to bend and are easier to break. If you will have to be making some tight corners the toslink might not be ideal. Coax is of course more flexible.
Second, coax is slightly more influenced by interference from other electrical fields. Still, you would need quite an interference to disrupt the signal. But it does mean that over longer distances and through possible magnetic fields the toslink is supposed to hold up better.