Optical Isolation

Technically coax should be better, since transitions are faster, hence system noise (jagged edges on transitions slopes) does not convert to timing jitter, but coax is prone to reflections on characteristic impedance boundaries (that adds/changes shape of transitions) and transfers digital noise by direct connection of the grounds. It can also create ground loops.  Optical is free of all these problems but transitions are likely 10x slower making system noise to convert to timing jitter easier (noise riding on the signal = jagged edge = variation in time of the threshold point).  If your system is electrically quiet or your DAC has good jitter suppression I would try optical first (that's what I have) - less hassle and cheaper.  Otherwise you need to compare different coax cables.  One with closest impedance match (not necessarily the most expensive one) will sound the best - cleanest (since jitter converts to noise).  Also 1.5m-2m can sound better than 1m (because reflections coming from the end of the cable, if any, will likely miss/be late the edge that started them) 

Any device, like streamer or WiFi receiver (like my Airport Express) should receive just bit perfect data, but can have different quality of electronics to convert this data to music (to add timing) and output it (strong, fast drivers, quiet power supply etc.)  IMHO it is impossible to express it in numbers - you need to listen.

@navyachts Jitter converts to added noise proportional to signal level (no noise without music), detectable only as a loss of clarity.  The best way to compare two streamers, network players or digital cables is to listen, paying attention to clarity of the music.