Eugene - my experience with the down v. air pads needs some background information first. I backpack in the Pacific NorthWet, usually above 4,000', I'm trying to get by with a 20 degree quilt and not a full bag year 'round, and I'm a cold sleeper. I always use a 1/8" GG ccf under any pad to protect them from poky things and add a little more insulation. I got the down and air pads in April, so I haven't tested the down pad on snow yet, mostly because I'm running out of friends who will do that. I may just go out by the side of the house and dig a snow trench and try it. I'll call it research when Mr. B starts scoffing.
I used the air pad in August and half of September and was much more comfortable with the wider top and longer length than with previous Neoair and Thermarest pads I've tried. So comfort-wise, they win.
Then I switched to the down pad at the end of September because the nights were getting down into the upper 30's and 40's. I did not do anything different with my clothing or quilt and was warm all night. All of this is subjective and with limited experience, of course, and the real test will be sleeping in 15-30 degrees on snow. I'll report back when I've got the data!