RE: Air Mattress or Cot?
IMO the cot will be more comfortable, however, you will still need a mattress on top of the cot for max comfort. Yes, the ground is colder, but with a cot, you have cold air under you, so you still need a layer of insulation on the bottom. Your sleeping bag doesn't help much because you are laying on it negating its "loft" which is what insulates you.
When I am sleeping on the ground, I use a thermarest, when I am sleeping on a cot, I use the thermarest on the cot.
I have never liked real air mattresses because they "wobble" too much.