The trolley vs van mount issue isn't about what anyone here thinks is best,.. its what suits YOUR round best.
A trolley is great if you have lots of areas where it would be hard to drag a hose, eg. through houses, across busy pavements, or where you simply find it hard to park close enough to where you're working.
Where these problems don't exist, a van mount is a lot easier physically,.. no lifting heavy barrels or dragging trolleys in & out of the van etc.
My personal advice would be to look at your round carefully & if you are planning to work less now, start by dropping houses where you have bad frames that wouldn't suit wfp, where you have to go over flat roofs or where access is awkward. Refine your round to suit van mounted working, as it is far easier on the body. If you invest in a good lightweight pole like an SLX, use microbore hose etc it will revolutionise the way you work. Even if you want to do the same number of houses each day, you will be at work for an hour less at least.
I'd also recommend doing the bottoms WFP as well. Switching methods & carrying 2 sets of gear will slow you down no end, and confuse your customers. WFP when its done right does give the same (or better) level of cleaning as trad, and while some customers might initially be doubtful, the results will speak for themselves.
If you can spend a couple of days with experienced WFP cleaners (Maybe 1 trolley user & 1 van mounted) you will learn a huge amount & it might help you be more confident in making these decisions. I'm sure there's plenty of guys here that'd be happy to bring you along for a day & let you get a more rounded view of the various wfp options.