Which is better Coax or Optical cables

""But" if a person were to place a DAC between the Oppo and integrated there would be a need one."

True, however it’s not as simple as adding the DAC and cable. More often than not it becomes an exploration to find the right combination that compliments your system. Have you considered other paths for upgrade?

Given that the contrasts between cable types has been addressed, and that you currently own only a 105D, the question becomes: Are you considering the purchase of a DAC and for what reason? 

Both can be good option and as advised by ahendler if you are choosing optical buy glass and not fiber,If you are buying
Coax buy a professional cable and not the cheap stock ones.

I am open for other options but I previously had a Exogal DAC between the Oppo 105D and the integrated and it was outstanding.  Had to sell it to raise some money but I am planning to do something in the next few months.  Just raising money and researching....

When you had the Exogal did you use coax or optical. Did you keep the cable when you got rid of the dac. Then use that cable firstt if you get a new dac
Alan