It's not so critical, Brian. A large extinguisher contains 5kg of CO2, the specific gravity of CO2 at room temperature is roughly 2kg per cubic metre. So all the gas in one extinguisher will fill a room of approx. 2.5 cubic metres, and it is heavier than air. So unless you are operating it in a 1x1m (40x40 inch) large "room", there is zero risk of suffocating. You don't have to flood the whole shed with CO2 to extinguish a small fire, it is enough to withdraw the oxygen in its immediate proximity. Once it has spread on a larger area, I suppose your chances with a powder extinguisher aren't too good either.
Mind you, there is also a huge difference in quality between cheap and not so cheap powder extinguishers. I have some here from reputable manufacturers that are almost 20 years old and still passed the (biannual) tests without any trouble whatsoever, and some cheapo ones had to be rejected at less than half that age because either the powder had gone hard or pressure was lost. One was rejected because it was more than 30 years old, but the powder was still okay and pressure was fine. Good old craftmanship
Then I ahd this other cheapo CO2 affair with a hand-wheel valve that was withdrawn from circulation because two of them had exploded in use and injured the operators...
A good 6kg powder affair is roughly 100-120 Euro here, while a high-quality CO2 unit is about twice that price. Money well spent imho, given the longer life span and the fact that you would really really want them to work if needed!
Just my 2p worth, you pay your money, you take your choice.
Cheers!