co2
If you possibly can use a scale to determine before and after weights (a simple hand held fish scale for example) to see just how much you are paying per lb and really getting in the tank. Depending on the conditions you very likely will not obtain 15 lbs of gas in a 15 lb tank for your $13. I use a liquor store to fill my 10 lb tank and they simply use a larger tank to fill my smaller tank not a force fed system so I usually get ~7lb of gas.
$16 in Houston, and air tools work great, particularly impact wrench, since its in short bursts and doesn't use my co2, cut off tools use a LOT of time and CO2, filling tires use quite a bit but a 15lb tank should get the job done for several trips out, I will say if you have the space a 20lb tank is much easier to get filled and costs the same to fill as a 15.
what do you consider "a period of time"?. with a 10 lb tank i can air up 3 times change a tire with an impact wrench and blow up an air matress. that equates to a solid weekend of offroad fun. i pay $8.00 for a fill up. also, i carry a 2.5 lb tank for reserve air which will air up 4 35s once. when i camp, i dont usually conserve. i'm always blowing something off or filling someone elses tires or blasting my son with air.



