Well there have been all kinds of good reasons given for feat selections. And Maul materials are running pretty close to even with a slight edge for Cold Iron.
While good reasons have been given for the various materials from a gaming standpoint, what about justification from a campaign standpoint?
Given that this is a pretty vanilla campaign (though quite fun), would Max's background be more likely to work with one or the other material? That is, is a frontier village/barbarian encampment more likely to be wiped out by something vulnerable to Silver or vulnerable to Cold Iron?
This is a question that has 2 important factors: 1) which vulnerability is more common among something that would/could DO that, and 2) which material would be rarer within that community (so that it couldn't be used to save the village)?
My personal feeling (at this point- feel free to convince me otherwise) is that either a Lycanthrope or a powerful Undead creature (like a vampire) would be more likely than an outsider or fey, and Silver would be relatively more rare than Cold Iron in a small frontier settlement.
(Then again, it just takes one idiot fooling around with a Necronomicon..."Klatuu! Berata! Nicrmmrmmrrrr...")