I did simultaneous comparison of adding micro-spheres to both regular epoxy and Smurf Snot or System 3’s  Gel Magic as it is officially known. The Smurf Snot is far far better. First, the spheres mix in much more quickly; probably because it is already a sort of filler. The spheres extend it and for fairing, make it easier to sand. Next, it is much easier to spread thin and with a smooth surface. The spheres in regular epoxy are more ragged. I do use a mix of about half spheres and half Microlite. Pure Microlite is too fragile in my opinion, and it shrinks a bit.