If the 20w DeWalt is cheaoer than the 18w Mil, which is the better deal?
To answe OP:
I use power tools daily. I make a living doing such. I recommend for impact drivers, in this order, and this is just me so YMMV:
Milwaukee
Ridgid (Almost all I use due to an awesome warranty program)
Makita (possibly above Ridgid, but without the warranty)
DeWalt or Bosch
Everything else LOL
Growing up I was taught that DeWalt was amazing. It was, and still is pretty good. I own some DeWalt tools. Since I've switched primarily to Ridgid on the recommendation of 3 coworkers I haven't looked back.
Again, just my thoughts. YMMV.