Have the 12 by 12 with wood burning stove. Camped out on a 2014 South Dakota Black Hills deer hunt 50 degrees at the beginning and -20 at the end of the hunt. Wood burning stove kept us plenty warm if someone would get up every couple hours to stoke it.
Thinking about adding a gravity pellet attachment to the stove and dump a bag of pellets before I go to bed. -20 is pretty cold at 4 a.m.