TA Cleaning,
I've recently been through the same thinking process as yourself.
My factors were:
Budget (a tight one)
The type of work I have.
My round is 95% estate residential, many with difficult access to the rear, and to the front in some places too.
I think a van mounted system would be too large for my type of work, and I'd be forever moving it down a street and possibly I would have parking problems too. I don't have the funds to get a van anyway!
Therefore I needed a versatile WFP system; so I chose a backpack. I also have a 50 litre trolley system (as yet unused) and my water carrying capacity isn't good.
So the next step to get a tow bar and trailer. Wor Lass will use what-ever system she can manage; I'll use the other and hopefully we'll both go full-time WFP within a few months.
At present I plan to do 'tops only', with Wor Lass doing the ground floor traditionally and on days when Wor Lass doesn't work, I estimate I should be to carry another 200 liters with me (joke - please don't tell her).
Oh, I'm also banned from talking WFP stuff in the house. She says I'm doing her head in.