The majority consensus (although not universal) is that optical S/PDIF adds another "layer" of conversion - from electrical to optical and back to electrical. Add in the fact that the typical TOSLINK transceiver is a very low cost part designed for inexpensive consumer audio.
While I sometimes can't discern a difference between cables of the same topology, in most of the cases I preferred the coax connection to the optical connection. The sound seemed to be more focused and solid. If optical is the only choice for a given component (like an Airport Express), then optical is the best!
If you decide to try an optical cable, I would get one of the
Silflex glass TOSLINK cables ($69). No audiophile BS, just precision industrial manufacturing. You could also buy an inexpensive ($25) RCA coax cable made with Belden 1695A wire from Blue Jeans Cable or Ram Electronics. If you feel the inexpensive coax connection is superior to the very good optical cable connection (and I suspect that you might), you can later investigate a more expensive coax (with return policy) to see if it makes a difference in
your system.
Steve