It depends on what you are wanting it for. Vehicle fire or 'house' fire.
Standard dry chemical extinguishers are for living space fires. They use ammonium phosphate as the suppression agent; very corrosive to any reactive metals it comes in contact with, including electronic connections. And it is very hard to clean up; gets into every crevice, nook, and cranny. They are cheap.
Halon extinguishers (Halotron 1) are used for vehicle and electrical fires; don't damage electronics.
Expensive.
Vehicle fires spread fast so you only have a few seconds to put an under hood or under dash fire out. Vehicle fires get very hot very quickly.
A 2.5 lb extinguisher will give < 10 seconds use.
A - ordinary solid combustibles like wood and paper
-- 1A = equivalent to 1.25 gallons of water.
B - flammable liquids and gasses
-- 1B = 1 square foot of class B fire
C - energized electronics
-- C = does not conduct electricity; its just a class A or B fire with electricity added
D - combustable metals
-- very specialized, usually targets a specific hazard (magnesium, titanium, or sodium).
K - oils and fats
-- found in commercial kitchens, another unique specialized extinguisher.
For dry chemical extinguishers in mobile applications you should invert the extinguisher and hit the bottom a few times with a rubber hammer once a month. This will break up the powder before it clumps. Once the powder clumps the extinguisher is useless and must be replaced. For stationary applications twice a year should be adequate.