This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
There are now three kinds of digital interconnects that connect
transports to D/A converters: coax, plastic fiber (Toslink) and
glass fiber (AT&T ST). In theory, these should sound EXACTLY
the same (bits are bits). However, this assumes good circuit
design (in particular, the clock recovery circuits of the DAC,
and careful consideration of electronic noise) which may be
compromised because of cost considerations or ignorance. Note:
different signaling schemes are used on plastic and glass fiber.
In any case, some people claim to hear a difference;
of those who do, most seem to prefer the glass fiber.
However, the technology of fast digital data transmission
in consumer electronics is evolving very quickly now.
Any specific recommendation should be treated with
suspicion until the industry matures.