One-size diapers seem to be the best economical choice. They literally grow with your child rather than having to buy several different sizes of diapers. That meant that our baby who was six months old could wear the same diapers everyday that our three year-old could wear at night. No more buying pull-ups, no need to buy large diapers and separate small ones for our skinny baby. Also, if we have more children these diapers can even be used for our newborns since both of my babies were larger (9 pounds 2 oz & 8 pounds 7 oz) and other babies most likely would be around the same size. I've even had small stuffed animals and dolls "try on" these one-size diapers. They actually fit!
There seem to only be a few downfalls of one-size diapers. Since babies come in all shapes/sizes, it is possible that there might be times when a one-size diaper doesn't provide the best fit. Also, since one-size diapers are used for an extended time & sometimes with multiple children, they receive more wear and tear than a typical sized diaper. However, even after both of those cons, a one-size diaper was the best fit for our family's diapering needs.