I did some research on optical cables a while back and from what I understand the difference between a quality cable and an okay cable is amount of jitter. Not sure if *I* can hear jitter with my current playback system (doubtful), but there are those who say they can.
If I recall correctly, more strands in the fiber = less jitter, and glass optical fiber introduces less jitter than plastic fiber. Check the Archival Info forum for a thread with some quality (but inexpensive) optical cable recommendations. I went with the Sonicwave, I believe. Check the post, I think I noted which cable I bought.