I ended up going with Green Mountain Guns in Lakewood, for convenience sake. I also wasn't impressed with the customer service at Prairie Arms. Green Mountain charged $25 and has a friendly staff.
I decided to go with the K3B with fixed carry handle and A2 sights. After a week of waiting for it to ship I emailed Atlantic to make sure I hadn't screwed up the order. They apologized for the delay and (completely without prompting) upgraded me to a flattop upper with removable carry handle.
Also, I fell victim to the Obama gun run and it ended up taking over 6 hours for CBI to run my "instant" background check. Fortunately, I was able to get to the range on Saturday and I was very happy with the rifle. My new PMags don't drop free from the mag well, but other than that I had no issues.
Thanks for all the feedback on Atlantic and Denver FFLs.