This is my opinion
Backpack - this has many advantages over the Trolley.
1. You can move around with relative ease........
Okay, there is a weight issue. Filling the 18L tank up to capacity, and then putting it on your back does prove to be a wee bit on the heavy side and awkward especially if you keep moving about (the water is pushing and pulling you about counteracting your movements), but once you have cleaned about 5 houses you get into a rythym.
As long as you have secured the top and bottom straps so that it fits to your body and you stand chest out-ish, there shouldnt be many problems. Yes if you have back or arthritis problems its not advisable to put it on your back. I am 24, 6ft, and 9st 11 (slightly underweight), and at times my left shoulder starts to go, but you persevere. Buying a sack trolley or similar is advisable, but we will have to look into its benefits.
For getting to awkward windows, where there might be plants, sheds, furniture, etc it is much easier to use a backpack because you only have to think about the pole and the microbore hose getting caught on something (but this comes with playing about with your set-up).
When you find the correct set-up its a doddle. Sourcing all the correct bits and finding the happy medium will take time. Without a place like Cleanitup we wouldnt have been so positive about window cleaning.
* WFP has a long way to go before becoming the standard way of cleaning windows. There are many stupid human beings preventing this. People have to be educated! *
Trolley
The only good thing about the trolley set-up (Freedom) is the fact that the water container is larger @ 25L, but as far as moving the bugger about its just a big performance........getting in and out of our van (Vauxhall Astravan) is a b'stard because it doesnt have barn doors.
I just cant see past the aggro.