I've found that these fiberglass products work well for a number of different purposes.
I use Dyna-Hair (long strand) for areas where I need strength over a larger open hole or to stabilize an area.
I use Dyna-Glass (short strand) on areas that have been covered with foil tape or where a hole is small.
I use Ultra-Glass (milled glass) on top of welded seams to create a barrier against potential moisture problems.
The longer the strands in the paste the more difficult to make the surface smooth during application. When you apply the milled glass it can be smoothed and pushed into the surface imperfections. Long strand will have air pockets and need further filling to get a smooth surface but the longer strand is stronger over unstable material.