I realize this doesn't answer your question, but one big problem is simply taking care of them. People that don't wear glasses (I think) have more of a tendency to abuse them. I started wearing glasses a couple years ago, and my first couple years saw me destroying several pair a year. I wasn't used to having to take care of them. After spending a lot of money on glasses and having to use scratched glasses while I was witing on another pair has mede me a lot more careful.
Shooting glasses are usually cheap, but this doesn't mean you don't need to take care of them (as you found out). You need to keep them in a case. You need to be careful about how you clean them................... Otherwise, you damage them and they are useless.
In direct response to your question, I would say that you need to get some made of actual glass instead of plastic. It is much more resistant to scraching, although you can scratch it pretty easily.