As a volunteer FF, I can affirm that water is really the way to go. Better yet, buy a compressed air/water extinguisher, and some class-A foam mix, and make a couple percent foam/water solution. The foam causes the water to lose it's surface tension, and it will soak into the wood faster, stopping the spread to unburnt wood faster, and then the water will cool off the fire itself.
These are the only extinguishers we carry. We use dirt for flammable liquids, much easier.
What I can't comment so readily on is the fire cause. Sources I can easily see are:
- sawdust lit by a cigarette or spark from "finding" a nail/screw with a saw
- pile of oily rags in a hot, enclosed shop
- electrical fire (overloaded circuits)
less so:
- flammable liquides/vapors
The first list is mostly handled well by an A extinguisher, but not if electrical is involved. There you either need GFCI (which you probably need in your garage shop in the US anyways after the 1999 NEC, and I'd still have a master switch for the room that kills everything but lights, and hit that before grabbing the extinguisher.
Most fires I've been to have been electrical in origin, or heat caused by faulty wiring/ventilation of something electrical (one propane leak was "fun"). Don't forget that thick sawdust (or plane shavings) + hot lights = fire.
I'd get both. Use the water for non-electric stuff, and the ABC for anything that might be live.
-Aaron