Which is better Coax or Optical cables

As @kijanki already pointed out, it's system dependent. One benefit of optical is the inherent electrical isolation. Lifatec makes excellent glass cables capable of handling 24/192 files.

http://www.lifatec.com/toslink2.html

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

""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.