Fiberglass with a tighter weave that uses smaller fibers and more per inch can save some serious weight. It's usually a satin or crowfoot weave. The weight savings come from the reduction in epoxy. The glass itself weighs the same but there are less air voids which need to be filled. The surface is also smoother saving the number of fill coats. For example: A 3.7oz glass that has 24x22 threads per inch (style 1522) is 0.0059" thick. A 3.7 oz satin tighter weave with 60x52 threads per inch (style 1165) is only 0.0042" thick. That's 28% thinner, which will save at least 28% in epoxy. The ones I've worked with seem to really like conforming to tight curves.

The negatives: tighter weave glass can be tough to wet out clear. The open weaves naturally have more voids for the epoxy to pass through. The resulting composite will be slightly less abrasion resistant. The larger fibers in the open weaves seem to take abrasion better and there is less epoxy to wear through.