Cordless impact wrenches are becoming more and more popular, but as far as power, reliability, and usability goes, there's really no beating a good quality pneumatic one.
Provided you already have a compressor, an impact wrench with the same power and quality will likely cost hundreds less than its cordless equivalent.
But even if you don't have a compressor, if you do a decent amount of home improvement, an air compressor may be worth the investment as you can use it for nailers, sanders, drills, and a lot of other great tools.
An air tool gives you a much better power to weight ratio than an equivalent battery operated tool.
The design of air tools is simple too, so your reliability goes way up.
You don't need a battery! You don't have to stop working because the battery dies.
So long as your compressor is good enough for your tools, you can work all day long and take breaks when you want, instead of when you realize that you forgot to charge the spare battery, and the battery you're using is quickly dying.
Cordless impact wrenches are much bigger and bulkier than air wrenches.
Primarily this is because the power source is stuck to the bottom of your wrench, instead of tucked away in the corner of your workspace.
If you're just taking off lug nuts this isn't that big of an issue, but if you've ever got to get into a tight space (like when you're working on an engine that's still in the car), the extra room you have with an air impact wrench will always be welcomed.
Another issue is weight.
Batteries are heavy! Batteries are getting lighter, but they are still quite heavy (especially if you need an wrench with a lot of torque...
high power = big heavy battery).
Overall, unless you need an wrench to carry with you in your vehicles in case of an emergency, or if you just aren't going to be doing the work close to an air compressor, an air impact wrench is the way to go in terms of reliability, power, and usability.