In addition to the hideous mess you refer to the dry-chem extinguishers will
destroy any electronics the powder comes in contact with -- may be a moot
point depending on the nature of the fire.

The CO2 is very effective on electrical (Class C), fuel/oil (Clas B), and
paper/wood/fabrics/skin (Class A) -- just about anything you'll happen across
in a vehicle.  Once the CO2 has done its job it goes away leaving no mess
other than the burnt stuff.   They are heavy, can give you a nasty little
static bite if not grounded, and you need to be sure you don't grab the wrong
part of the cone or you won't be able to let go til your hand thaws.

Halon is very effective on fires in a confined space.  Any amount of
ventilation will reduce its effectiveness.  In the open it is almost
worthless.  It is falling out of favor in the marine industry because it
tends to generate some very toxic gasses as it works.

Your best bet is to either use the dry-chem and resign yourself to replacing
all sorts of circuits or spend some time at the gym and pack the  CO2.

I agree with Gary that a good deed is a good deed and you should just enjoy
the good feeling and eat the $20.  Of course, I'm a Coastie and have made a
living doing good deeds for almost 20 years.

Chuck
'85 Westy (Roland, Son of Rollo (dearly departed '67 Bus)) 45k
'89 Honda Civic Wagon (The Timex Car) 116K