I suggest a backpack, worn on your back. Re-filling it is quick; but having the water handy re-fill it can be a pain; but just takes some prior planning.
I've just started using this method following someone else's advice on this forum and it's great. Carry spare water with you on a sack truck and leave it in a handy-place.
Only fill the backpack up half or three-quarters or only enough water to do a couple of jobs. Where-ever you can walk; you can clean windows with a WFP.
You end up just walking up to the windows, cleaning them and walking away; really quick. No messing with sack-trucks, hose tangles; steps etc.
When the backpack is full(ish), I carry it on one-shoulder (easier than the 'grab and go' method I used to use), and just dump it down at the front of a house. When it gets lighter through water usage; I just carry it on my back.
Today, I did my 'most difficult to access' accounts; back alleyways, overgrown with weeds, nettles and spiky things that scratch your legs.
The backpack was great on my back.
For any difficult access areas; you can't beat the backpack. Where you can get your body, there's no-where it won't go.
It's also great if you plan to do first floor and above only; given it's limited water carrying capacity, which I'd say is 35 litres (10 on your back, 25 on a sack truck).
The backpack does have its limitations though.
What system you use depends on your work.