Right you are, Mister Stick.
Larger tanks are more economical to fill. I just don't like paying rental fees. The 40cf tanks allow me to own the tanks and when I swap them for filled tanks I get tanks that are tested and certified.
I am in the process of trading 2 20lb. co2 syphon tanks (used to fill paintball tanks) for an 02 and an acetylene for spares.