I have been in both varieties a few times ie ...glass vs. aluminum ….a lot of it depends on the total weight and design of the watercraft.
that said glass is typically heavier in most cases . but again there are exceptions , my little boston whaler was like a Styrofoam cup and my little aluminum was less likely to blow off the lake and it was lighter than the whaler and both were about the same length and the aluminum was much lighter .