You can go ahead and buy a backpack, it will always pay it's way. A backpack is very handy for smaller stand alone jobs, but I don't think it's up to prolonged all day contract work.
If your any good at DIY, you could build yourself a trolley system that would carry 2x 25ltr containers.
By strategically placing your containers before hand, it would do the job as well as any van mount.
It all depends really on what you can afford to pay, and what size vehicle your working from.
Either way the DIY route could save you a lot of money, and there's plenty of help available on here and the DIY water fed pole site.