Bummer on the avalanche and awesome that he is OK. I was in a big one in 2004 and it is not fun. I wear an Avalung which I think has more value and costs less than the avalanche airbag systems. It will work even if you are stopped at the bottom of a hill and get buried. The airbag won't do you a damn bit of good in that situation. I also am not sure that I want to be hauling *** down a tree covered mountain with the airbag holding me way up on top. If you hit a tree it is going to hurt!
If the trees are on the top and your hitting them you would be hitting them under the snow. I would rather be on top than under the snow. If you were buried deep enough the weight of the snow could crush you. My vote is for the top. The avalung is probably a good product if you get it in your mouth. Anything to increase your chances is a plus. These guys got air to this guy in a few seconds because they had a visual. Not going to get out that quick if your five feet under.