There are a few key differences between vinyl and fiberglass windows. Firstly, vinyl windows are made from excreted PVC (polyvinyl chloride) as the base. The main benefit of vinyl windows is they are one of the most affordable window frames. They are also highly energy efficient and can be made to fit any opening size. Vinyl windows do not need to be painted, and come in a limited range of colors.
Fiberglass windows are made up of a composite (a mixture of fiberglass and polyester resins) that can be much stronger than vinyl windows. However, fiberglass is more expensive than vinyl, and depending on the brand can be significantly so. The glass fibers in fiberglass windows mean the material will not expand and contract during extreme weather, making it a better insulator than vinyl. Fiberglass windows can also be fabricated to look like wood, while also offering more color options.
You can read more about the difference between vinyl and fiberglass windows here.