Have always used the shorter pads, around 4 feet, to save weight.
Currently, the Nemo Zor because it is the lightest, is more comfy than the Thermarest, and got a good review from Roger for R value.
Can't sleep on air mattresses of any kind - they drive me nuts - like floating on a bubble. Good on those who can.
More useful than a pad to go under the feet are thinsulate surplus booties that keep the feet warm where there is no pad under them. They are especially helpful when my sox are damp, and get used at least half the time. Yes, in a pinch, the pack, rain jacket, or whatever can go under the feet.
The Zor cushions from my shoulders to just above the knees. Am a side to side sleeper.