I always get glass eyeglass lenses. They are more scratch resistant, and the optical performance is better, especially at the periphery. They are worth the extra cost and additional wait time, in my opinion.
A few years ago my frames broke just before I was leaving for a business trip, and I needed new glasses on short notice, so I got plastic, which is lighter, less expensive, and more readily available. The color fringing at the margins was quite noticeable, as was the loss of sharpness. With my current glass lenses, made by Hoya, I can use the whole lens. I find this is particularly important for reading music, where it's not unusual for me to have to be able to resolve fine detail at the right end of my peripheral vision, pretty far off axis. When I start missing notes in the lower right hand corner of the page, that's when I know I need a new prescription.