I've gone with and without a cot. For me, a good air mattress is *essential*. Also essential is a good wool blanket or two between your sleeping bag and the air mattress, the condensation is pretty bad when it's cold. I've never noticed whether the cot was colder or warmer, my down bag always kept me warm enough, put your clothes down by your feet, don't wear them. I've done this down to -14degF.