Are you looking to add an air impact wrench to your toolbox? The most expensive, and the most powerful, is not necessarily the right one for you. 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).
With the recommended torque, you will want to add an acceptable safety factor to figure out how much torque you will need. If you get an air wrench, typically they will give you a max torque, and sometimes you will get a working torque range as well.
First, you will want to add a safety factor on top of the manufacturer's proposed torque, to account for the environment that your bolts will be in (I'd recommend you take a torque wrench out to see how much torque you need to loosen a "worst case scenario" bolt to get a realistic number). You'll want your impact wrench to be able to easily handle that "worst case scenario" (no one wants to bring out a breaker bar because their brand new torque wrench is not powerful enough), so make sure that this is easily within the "working torque" of your wrench.
If your prospect wrench only has a max torque spec, you'll want to add an additional safety factory to account for the lack of information. Try to find an impact wrench that has working torque information, and is in the same price / quality range, and see what it's max torque is, and find some comparable. This obviously is not an exact process, but it'll give you an idea.
So there's a quick and easy 3 step process to determine what you need from an impact wrench. Figure out where you'll be doing the work, what size nuts and bolts you'll be working with, and how much torque your impact wrench will need.