My exact question.
If you think about it, at the same tire pressure, you're going to have the same number of square inches of rubber on the ground.  The only thing different will be the shape of your contact patch.  Would you do better with a shorter, wider contact patch, or a longer, narrower contact patch.
I'm thinking that for mud, shorter and wider would be better, and for rock crawling, longer, and narrower might be better.
EDIT: And look - my thinking is exactly OPPOSITE of Jason's. Oh well...