Okay... HONESTLY, there's no difference in quality between the two.
Some will debate until they're blue in the face that one is better than the other, but the sheer fact of the matter is that they're both DIGITAL. The SPDIF protocol has error correction and the ability to retransmit lost data.
Since the data arrives in digital form, if it's degraded or incomplete, the receiving device will request a retransmit.
The argument is that since coax is using an electronic pulse to send the data, it is susceptible to EMF and other forms of interference. Where as optical uses fiber optic and light, so it is not as prone to this type of interference.
The fact of the matter is, they're both DIGITAL. So the quality will be the same on either.