Don't worry about it, for all intents and purposes they are equal...
TOSLINK is an inferior technology, period. Yes, galvanic isolation, but it has absolutely no other redeeming values.
Coax outputs can be good or bad. Slim Devices (as an example) stick ferrite beads in series with the coax output. This helps them to pass radiated emissions standards, but mucks up the sound.
So, any comparison between the 2 may not exactly be all that it is cracked up to be.
If all that you want is music, then TOSLINK will work. If you want something better, coax is the only real solution. But, not all coax outputs are identical, or even very good.
Ar_t is correct, Toslink causes the most jitter of any of the digital interconnects, primarily because of the conversions from electrical to optical. However, this is not the only jitter issue. The Duet jitter is still there. This can only be removed with a reclocker. The reclocker can also change the interface from Toslink to AES in the reclocking process. Then the Toslink from the Duet does not matter. Then you can use the AES input on your DAC, assuming that it has one.