My recollection from building a fiberglass plane is that the micro-balloons are good for thickening the slurry for filling voids and making bonds that were not stressed while cotton flox was called for when it was a bond that needed to be strong. I used flox on that bond only because the slurry with micro-balloons tended to flow and run away from the two surfaces. With flox it was more pasty and didn't run away from the two surfaces but either would work on that joint. And, just in case there is a difference between the glue types, I was using 3-M rather than Hysol.