If it's really cold, you use the close cell matress, and you get a nice ergonomic shape in the snow. Moderate cold, and I use the CC also. On hot days It probably doesn't mater.

The stevenson folks started the down air matress thing, as far as I know. Their classic ad showed their integrated bag and matress unit floating on a lake. At least they were smart enough to get rid of the bottom half of the bag. If you use a synthetic bag, you are proabaly well enough insulated anyway.