A useful alternative happy medium between a backpack or van mount would be a trolley. No carrying the water, (just pushing it on wheels for the whimps, (that's me).
Personally I prefer a van mount system but if the need ever arises I would chose to use a trolley for the odd, awkward job or a backpack on wheels.
My last backpack I used, I through over a balcony 4 storeys up because it was so frustrating working so slowly with having to keep filling it up and sorting out flat batteries and pump problems. That day I'd had the last straw and through it over the balcony. It was the best move I ever made but I don't recommend throwing them from height like I did because they never work again,lol.