Like Slayer said, the Milwaukee M-18 brush-less with 750 in-lb of torque is all you need. Paired it with either a Mora or Laser bit and you will be happy forever. They come with Lithium Ion batteries so all you need is 2. They should last you all day long.
What brand is your 18v? I used the old blue 18v Ryobi for the first year and it did fine with 2 2ah batteries. I have the Milwaukee Fuel now, and it's amazing, but not cheap. THe new ryobi's (neon green) are not built as well as the old blue ones and some folks have had issues.
I'm hearing mixed reviews on the rigid...def plenty of power, but some mechanical issues. I think no matter what you go with, 2 batteries is a good idea, 7" auger max and 3ah or better batteries.