The two main differences between the flocked (crushed bristle tips) and the mono filament (straight) are the flocked is kinder to wooden framed windows but entices some old black sealing rubbers that have oxidised (perished) from south facing windows to bleach down from the top seal onto the glass and bottom seal onto the window sill if using 'brush on' method when rinsing unless you lift off but that's making life harder for oneself.