Co2 for me every time, you can use multiple bursts to extinguish re ignited fires, it makes no mess, it may asphyxiate you but not as quick as the fumes from the fire! Keep it by the door - that's where you want to be headed when you are going to get it. If you have a smallish shed then cut a hole in the wall big enough for the funnel and plug it off, then you can get out, shove the funnel through the hole and fill the entire shed with co2 - as long as no one remains inside. It will extinguish fires in cavities, engine bays, dust collectors, flues, chimneys and other spaces where you can't see inside, very quickly. On fires fuelled by wood, rags etc. you can keep it by you while shovelling the embers up and use it again if needs be.
You can even use it to freeze a bearing when preparing for a shrink fit. Oh, and it is very eco friendly - no nasty chemicals.
