the 2 best cordless 1/2" impacts money can buy:
Ingersol Rand w7150
Milwaukee 2763.
I have the milwaukee I use for changing tires and other impact duty, it works phenomenally well.
visit the garage journal for the reviews on the two...im a big fan of the milwaukee FUEL cordless tools!
air tools and cordless tools have their place, I say buy both, you wont be disappointed.