Fiberglass is relatively new for this particular application and not yet widely available. They have the highest R-values of all frames; thus, they are excellent for insulating and will not warp, shrink, swell, rot, or corrode. The sashes can be made in a variety of colors and can hold large expanses of glass. Some frames are hollow; others are filled with various forms of insulation.
These can also be painted which provides some flexibility if you wish to change the color of your new windows in the future.
However they will be more expensive than products made of aluminum or vinyl. In addition, as they are not yet widely available on the market the array of sizes will be limited. Still there has been much good advancements made in the mass production of this type of option and many homeowners have been very satisfied.