Something you should be asking yourself is what type of fire are you most likely to come across? Are you worried about a spark lighting a garbage can of rubbish on fire? Or having a combustible liquid catch fire? Or something electrical where you are unable to de-energize it? If you are planning to use it on a vehicle, your best is probably to get an ABC dry chemical. This would best cover you off for each of those types of fires. Yes dry chem can make a huge mess with the powder all over the place. However is that mess more or less of a problem then the shop burning to the ground.
Lastly one needs to consider the size of the extinguisher. The smaller ones will only last a few seconds and need to hit exactly on the seat of the fire. Otherwise you are wasting your time. In my shop I have a 10lbs ABC on the wall by the man door. But I also have a fire stn across the street where I am a volunteer. If I didn't I would have a 20lbs extinguisher.
This is a great website for explaining all the types of extinguishers and their limitations.
https://www.ccohs.ca/oshanswers/safe...nguishers.html