All I'll say on this one is "manual handling", just how much do you think 50 Litres weighs and how difficult is it to lift into place. My 40 Litre Aqua roll is plenty to lift. I use my original 29 Litre Aqua roll as a backup and that last's us three days even with shower use.
The longer the water is left stagnating in the summer sun the more chance you have of getting little bugs!!