You know what makes you safer than either a helmet or an avalung? An avalung AND a helmet. Of course you're going to get the "both" answer. There's no reason you have to leave one behind because you are taking the other.
How about you just take both to the top of the ski run and then decide which one you leave at the top while you drop in.
I think you're asking a question akin to "should I go biking with a helmet or gloves?" and the answer is "do whatever you want, but you're safest with both." Avalung or helmet? It's a false choice.
What you might want to ask is whether you should bother with an avalung and just save up for an airbag. I can get you the BCA one for a great price...