Glass is the cheaper option of the two, and although plexiglass is shatter resistant, it will crack with enough force and it's more easily scratched up than glass. If it's fixable and you don't care what it looks like, I'd throw some duct-tape on there and call it a day. If it's not fixable, I'd replace it with more glass, simply because I think the force of the rock would've cracked plexiglass too.