I have a trigger on one of my poles, on a standard window great, on an awkward window say where your leaning over a conservatory, becomes a pain in my opinion as its stuck in one position, so you constanly have to hold the pole in the same position.
My father in law managed to make a fitting so the trigger slipped up and down.
Also if you are doing a large surface area, its a pain to squeeze it all day,
i think a on/off valve on the pole is the best bet!