Fiberglass fenders would be lighter. The fenders don't have anything to do with the structural integrity of the car, just make sure you don't get the race weight fiberglass ones, they are a bit fragile for street use. Actually, I wouldn't be surprised if you couldn't do some digging and buy reproduction steel front fenders for about the same as the glass ones. Oh yeah, might want to check and make sure the glass fenders have provisions for attaching your inner fenderwells, too.