There is also "impacts per minute," that can affect how well it does the job, if the torque rating is the same or similar.

I don't have the 1/2 impact wrench...it's on my list though.
I do have the m18 and m12 1/4 impact drivers. With the adapter, they can really remove some big bolts. But, it is hard on them. The chucks get the piss beat out of them, as well as the adapter.