bacampe, the rule of thumb is that those small extinguishers can put out one small garbage can's worth of flame at best, and they will last less than 30 seconds. If you've never used one, ask your local FD or USCG if they conduct any demonstrations, you really want to experience the way they work. I've seen folks pull the trigger and then just stop--because the noise and cloud spook them. Or, use it from the wrong distance because they simply don't know the right one. It helps to actually have used one, or at least seen it used.
They also create a fog of yellow powder that makes it hard to see and breath, btw, so you want to use it and scram, whether it has worked or not.
We keep one at the side of the companionway, so it can be grabbed from above or below, and another in the v-berth just inside the entrance. A third by the mast in the cabin, where it can be reached by someone going either way.
Before you mount it, also consider what may foul on it (i.e. lines hanging in the companionway) or whether anyone may hit their head on it, or be thrown on it.
The fire blanket is definitely a good idea, they last forever and need zero maintenance, no recharging, and make no mess when they are used. A dry powder extinguisher will leave you with yellow "dust" in every corner of the boat for weeks after using it. And of course, the fire blanket doubles as a tea cozy in case your teapot doesn't already have one. (VBG)