Archimago has measured the various interfaces, and usb turns out to be best, followed by coaxial, followed by optical. The differences only seem relevant at higher resolutions. See here for some of those measurements (but dig deeper and you wil find more): http://archimago.blogspot.nl/2017/08/measurements-smsl-a6-as-dac-part-ii.html
Copper vs Optical Fibre
Can anyone answer me (especially the equipment manufacturers) why copper connected components can have signal clarity and capacity to 32bit/768kHz, DSD 128, 256 etc, while optical fibre connections (toslink and the like) are restricted to 24bit/96, DSD over DOP, PCM if you are lucky to them up-sampled. In Australia, the National Internet being rolled out has very high up/download speeds on fibre is you can get it, and lousy speeds on copper. Why is optic fibre not used more extensively (between components and speakers if possible) as it does not suffer the maladies of copper connections?