Now M12 and M18 refers to the battery power on the tools. M12 has a 12 volt batter while M18 has an 18 volts. The M18 is going to have a higher power output than an M12, however the M12 will have a much smaller footprint while working. This is beneficial as vehicles have many hard to reach bolts, and also the lighter wrench will make it overall easier to work with in long jobs. 

At the end of the day, there is no wrong choice between the M12 and M18 impact wrenches. It all depends on what the user is looking for. The M18 line will be better for hard, every day use while the M12 line has a more comfortable feel while working on the small stuff.