I know Casey, Art and I all use Silpak resin. I've been using their Siltool, pre-tinted black. It always comes down to preference thing. I find Smooth-on products are nice, but pricey. Burman's 808 was a bit to "rigid" for me. With siltool, I found a nice plastic that cuts and sands well, but doesnt feel brittle. There is no "best" ....just best for you.
As for glassing? ..agreed ..stronger and lighter ..but what a pain in the ass it can be. ha ..I only glass when I have to...usually for film work. ( molds, animatronic cores and understructures, and props)