We are talking about MADI? (because that’s the only expansion card for uTrack24 where you can use copper or optical cable)
From audio quality perspective, there’s no difference between the two cable.
The main difference is that the cable length:
Coaxial, max. 100 m (75 Ohms, BNC)
Optical fibre glass, max. 2000 m (62.5/125 µ, standard network technology)