CO2 pricing varies a lot from place to place across the country and even different places in the same city. When shopping, I found the off-name welding gas places, cheaper, next was name brand (Airgas?) and then specialty places like home brew or paintball. Near me, the paintball was priced at around $1 per pound, making a twenty cost me $20 instead of near $14. But the combo is what sold me as it was a question of taking it in, waiting while somebody got available to fill it against pulling up to a dock where they have the tanks, drop my empty and while I go in to pay, the guy loads the new one. But that all depends on lots of small details. If I was hauling it down the street on a bus, a five pound might fit better? Twenty pounders are semi-heavy when you get the tank and twenty pounds of gas added together, one might not want to drag it up the stairs to the third floor!