I would personally buy one like Xlines big orange one,,the reason being is you won't get air locks like you would on others where you need to change barrells,this used to drive me mad,get an Xlines 45 liter one,not the best looking ones about but holds plenty of water unlike the 25 liter barrells you change and run out half way through a job or when you have one window left to clean,either Xlines or something similar like the Brodex 50 liter thats holds plenty of water.
It's easy to add water using 25 liter barrells to top up,much easier and less frustrating than getting air locks after every barrell change.
That's my experience anyway.