The propane or gas version is cheep to operate compared to tanks. They do have a down side and that's heat. They burn several flames to make CO² but they do it on the cheep. Tank does cost more money the large ones last a few months and cost about 40 bucks to fill. Tanks do not have the heat problem and that's always a positive thing to not add heat. You only run CO² during light hours so the heat is at max anyway. Unless you have a very large AC system it's hard to run natural or propane gas to make CO². The warmer temperature could cause the plant to turn hermie so watch out for temperatures above 88F.