If you do fiberglass correctly it is no heavier than monokote. You have to keep the finish thin by sanding every coat to leave only what is absolutely necessary.

Also, as ho2zoo said, you can only glass sheeted surfaces. If you want to use paint, but don't want to sheet, then you can use tissue, silk, silkspan or coverall with dope. Doped finishes are also light and don't take as much effort to keep them light, but they're still a lot of work. In my opinion a painted finish is much superior to film, but they are so much work that I only paint "special" aircraft - especially considering I'm such a lousy pilot that my planes don't live too long.