I'm with js3 on this. Although the fibers are "chopped up", they provide a matrix that the epoxy interbonds with on a microscopic level, and gives the epoxy a much greater shear strength. Microballoons are microscopic glass spheres, and are used to expand the volume of epoxy,, lighten it, thicken it, and make it more easily sandable, but they add almost no strength to the mix. Think of them as tiny bubbles, and mixed with epoxy, make a "froth" that is great for making filleted joints and seams. Using either one will shorten the working time of whatever epoxy you're using, also.

When working with either the microballoons or the milled fiberglass, be careful not to inhale ANY of either. They will blow around like dust, and getting them in your lungs can lead to pulmonary edema and asbestosis-like symptoms.