Hi Joe, I think you have it in one.
We all go on about the best brush, but to get to the point of it

, its really the bristle and what sort you like for DIFFerent jobs. Shaving or yard brush for your face or car etc. Do you do frames, small windows or large, leaded or fascias. I have 2 dozen or so to chose from.
Splay means the brush wont stand off the glass like a wire brush or a brush with close flocked does and will splay into the corners and frames.