I use the 6-packs (of 0.5l) for car-camping. I use 4-packs of 2-litre for contingency storage at home. The larger bottles are cheaper, without being more than I think I can drink in a day or two. I usually drink about 1L per day, so a 4L bottle would be pushing it. I like shop-bought water because it has a use-by date marked, and because it's convenient.