propane without a doubt, just cus its sooooooOO cheap. i love it. you can just empty dump your mag if its getting low and give it another fill. Im using a 5 dollar bottle for a couple months and its not even 10% empty.
i used to have a paintball gun that used CO2 cartridges and it was a pain in the ass, they cost at least a dollar each! on top of that , if you only have 10-20 shots you dont really want to waste the co2 so you end up having to reload while in a firefight quite often. also they can be really hard to find at times