brush heads are a personal choice.it also depends on whether you use hot or cold water and what work your cleaning.
both the SL standard dual trim medium mixed and extreme versions are good with cold water but i found the extremes seem to look battered after a few months and didnt scrub as well.
my brushes of choice(now i use hot water) are extreme stiff bristle for most regular window cleaning work as they are so so light and still scrub well for a small light brush.
SL standard stiff version for very dirty first cleans and some other work.
radius sill stiff brush for UPVC/f/s/g and conny roofs cleans
SL flocked standard brush screwed on to a 90 degree gooseneck specifically used for solar panel cleaning.
these work for me at the moment.
regards
dazmond