I first started out using a trolley system which worked ok, the major problem though is if you only have 1 or 2 customers to do, the set up time can be irriatating, you then have the problem when you have more than 3 customers, because you have to kept on going back to your van to fill up.
A van set is a lot easier, both for the single customer and the whole close.
I have also used a backpack, but I found this was too much hassle with putting it on and taking it off, but last year I brought a backpack from window cleaning warehouse which you can carry instead of wearing on your back, this I found is an excellent piece of kit.
I personally would recommend a van mount system with a backpack for the houses where you have to go though the house to reach the back windows.
Trailing hoses can be a H&S problem but if you run the hose down the kerb side of the road and only go over the shortest point of pavement, you can then place warning signs of the danger of trailing hoses.
You also have to take into account that on a first wfp clean, you are going to use more water than you will when you have been cleaning these windows a few times, this will also be the case even if you have "trad" cleaned them in the past, because on the 1st clean you have to remove all of the dirt and grime that is in the airvents and framework.