Plastic or glass it doesn't matter, what matters is that is that the tip has a big surface area to let in as much as the light in as is possible and that the tip is as close to the light source, I've tested a few cables
ebay glass cable (sonicwave)
monoprice toslink cables
Dayton glass cable
Choseal plastic cable
They were all 6ft. The best ones were the dayton glass toslink cables and the choseal plastic toslink cable, which I received for free with a dac I bought, the brand was choseal but there was no model number on the cable.
I also own mini to toslink cable by choseal and it sounds very good but I've only compared it to the monoprice mini to toslink , the choseal is alot better.
What the two best ones had in common was that they had the largest surface tip area and also the tips were were a little more extended than the other ones so they were closer to the optical source and receiver.
I hear the lifatec glass cables are pretty good but I've never tried them.