Milled glass is cheap, that's for sure. It's $7.85/lb at CST. A pound is a lot of FG fluff.
http://www.cstsales.com/high_strength_fillers.html
The strength of FG comes from the tensile strength of the fibers. Chopping the fibers up kills that. It is a nice light weight filler, though. I doubt that there would be a big difference between the glass and the micoballoons. The main idea is to build the lightest possible fillet, since it's so far back on the airframe. Fillets won't provide a huge amount of strength, but some, certainly. Now I'm using West System 410 filler in their slow dry epoxy and getting the lightest, easiest to sand fillets I've ever had. I'd call it a toss-up. Use what you are happy with.