I'd snag them up personally. That's a good deal. They're steel, so heavy, aluminum is better, but you can take both tanks in and probably swap it for one aluminum or even a 5lb and 20lb aluminum (may cost a few bucks)
Sometimes hydro is free with the purchase of gas if you take it to the place on the sticker. I know airgas does free swap in or out of hydro for the cost only if the sticker says airgas on it.
Regardless, $30 for both is a hell of a deal. If the one on the left has no sticker one it you may have issues getting some to swap it. Make sure the valves are all correct too. CGA320 I believe is co2
Btw, 15 isn't too common, may have issues getting swaps with out calling in advance to assure its in stock.
I'd try to get a 5lb aluminum and a 20lb aluminum swap for free (the 15lb take is worth more than a 5 and a upgrade to aluminum) but not everyone will do It.
Then use the 5lb to dispense, and use the 20lb to purge and carbonate.