From Wikipedia "TOSLINK cables do not work well (and may even suffer permanent damage) if tightly bended, and their high light-signal attenuation limits their effective range to 6.1 metres (20 ft) or so. On the other hand, TOSLINK cables are not susceptible to ground loops and RF interference, like coaxial cables. Any standard 75 O A/V cable can be used for coaxial connectivity, while TOSLINK requires a specific cable"
As for do more expensive coaxial cables used as s/pdif digital audio connectors improve sound. I think not.
Results from testing free coaxial cable.
Phono cable length: 1.2 m
Nominal impedance : unknown
Measured Results:
Amplitude versus Frequency response: 6 MHz -0.12 dB. 100 MHz -2 dB. 330 MHz -3 dB. 800 MHz -6 dB
Frequency-domain amplitude ripple, with a periodicity of about 60 MHz, was about +/- 0.5 dB , presumably due to impedance mismatch.
Group delay versus frequency response: 500 kHz to 1.3 GHz: 8 ns average with +/- 1 ns ripple (again probably due to mismatch)
Cable frequency response is near perfect up to 6 MHz. S/PDIF is around 3MHz
Hifi component manufactures would not ship their components with cables that diminished the performance of their products. Freebee cables are perfectly good.