Back packs are the ultimate in portable, I find I can do 3 councle houses with them. They hold 18l of water.
Trolleys are the next stage up, these hold from 25-75l of water.
Then you have van mount, very good but not very portable at all.
I have a van mount and a back pack, if I can get close to my work I'll take the back pack. If you buy a trolly or a back pack you'll never be wasting your money.
As for companies the best is not necessarily the most expensive, all though they do generally have better after sales service.
Simon.