Just getting this back to the top. Window Cleaning Forum sure is busier than the rest 
You would think us window cleaners had nothing to talk about

OK sounds like you are asking for a little more info :

This is just my opinion but given the fact that you clean buildings I would still go for the van system :

Trolleys are great for certain domestic jobs or windows that dragging a hose is not good but other than that the van system is better
You will find that as you are cleaning you will need to keep going back to the van as 25lt is not a lot for cleaning buildings as well I found that I keep getting airlocks more than a van system
If you decide not to go down the van system do not be tempted by a backpack as they have a smaller pump and battery then a trolley and if you decide that later on you want to upgrade than with a trolley you can do that
One last thing I would ask the members on here what pole they would reccomend as this will help you..
I dont have a lot of three story and would only use a heavy pole for the odd one or two ?
Hope that helps
Adders