There are 3 easy steps that you should follow while shopping for one. Those include where you'll be using it, what size bolts you'll be working with most often, and how much torque you will need.

The first question you need to ask yourself is where you'll be doing the work. Will you, or can you be near an air compressor? Air wrenches are typically lighter, cheaper and more reliable than their cordless equivalent, but if you're not going to be anywhere near an air compressor, you'll have to go with cordless (cordless technology has been getting much better, but I do have a soft spot for air tools). This is the first thing you need to know.

The second thing you need to know is what you'll be using the impact wrench for most of the time. This means what size bolts, and what the manufacturers recommended torque is for those bolts. From here you can figure out what size drive you need by knowing what sized sockets you need (you'll want to get impact sockets as well, regular sockets can shatter when you use an impact wrench).