Digital Jitter is not caused by the physical properties of the cable (i.e., glass vs plastic). If the quality of the cable were to affect the signal at all, it would be an all-or-nothing issue - the signal either is there or it's not, basically a data error. This would manifest itself inaudibly as dropouts in sound. Digital jitter in S/PDIF would manifest itself audibly in the music.

The jitter introduced by the S/PDIF connection is a result of the signal itself, which is produced by a combination of the source signal and the signal introduced by the S/PDIF hardware at the source end.

In other words, an expensive ST-type glass toslink cable wouldn't solve jitter issues.