I would go for the 1KG one myself.
The discharge time of these is around the 10 second mark so imagine what your going to get out of the smaller aerosol types!
Also, make sure you know how to use it.
Most car fires start under the bonnet.
If this happens then DON'T open it.
Use the extinguisher through the grill or just pop it onto the first catch and shoot it through the gap.
Note that powder extinguishers work on a chemical reaction with the fire and thus have no cooling effect. Re-ignition is possible so keep the bonnet closed and dial 999.
I applaud you for thinking ahead. Every one should have one of these in their car.
Lecture over
Obelix