I've thought about using a fiberglass windshield frame but never bothered to replace mine. (the rust is under control and the pitting is minimal.) One of the things about fiberglass is that it requires extra strengthening so there is usually a metal frame inside so you might not be reducing the weight. Also keep in mind that a YJ and TJ use the windshield to support the front spreader bars that come off the main hoop. This is moot if you have a 6pt cage though.
With a little extra prep, a replacement steel windshield frame will hold up fine. The stock ones aren't painted very well from the factory. Pain is thin at any edge or corner. If you look behind the hinge, you'll see it's not even painted. They usually rot out behind the windshield gasket/spacer that goes around the glass and covers the gap. Mind started rusting out at the middle and the lower corners where fine dirt collects and causes moisture to rot the metal.
