Pureease ... I have used a trolley - Peter fogwills 50L (2 x 25L jobby) - a backpack - the 16/18L shurflo - a car with barrels, a car with a 125L plus barrels, a car with 175L plus barrels, and a van - 400L tank.
For me the van does the job and I don't use a backpack or a trolley at all. I run the hose thru houses from time to time and overall I find it quicker.
But... if I had to use the family car - which I did until I bit the bullet and financed a van then a trolley is a good idea. Personally I like the 2 x 25L of Peter Fogwill because you can choose how much water to take (25 or 50L) and it's quite light to use and put back in the car. The frame is light, the battery is easily removable and the barrels are usually low to empty.
I've never gone for a trailer but I think that is a solution too.
Overall for me a van is the best answer and if I had a job (say, internal courtyards) that demanded it I would go for a backpack or trolley - but only then.