Well if you have an airbag, in an avi I'd probably just pull the bag cord and worry about the valung afterwards if I had time. Bags keep you from getting buried in most cases- I find that avalungs would be most useful in a treewell or something like that- where you can't get out but need to be able to breathe, and an airbag would be useless.
Each tool has it's use, and if you can afford them I can't think of a reason not to have both.
"I know you believe you understand what you think I said, but I am not sure you realize that what you heard is not what I meant."- Alan Greenspan