the heavier slide wasn't made to make the gun more reliable or last longer or any of that it was simply to provide more mass for the .40 cal round, and therefore to help absorb some of the recoil.
FS is standard slide, with decock/safety, Brig is the heavier slide, Vertec is the new frame style with no hump on the backstrap and a light rail up front, and a different slide (same weight roughly as the FS), with a dovetailed front sight (Brig has this as well, but FS is a fixed front sight).
A G model is decock only, no safety, and a D model is usually a slick slide with no Safety or decock, and normally a spurless hammer.