I do both. I would say that early on skinning makes more money and has the bonus of getting xp from mobs that you skin. However, at +300 skill level, I think that herbs make more money overall.
Out of the 2, I would say Herbing. But overall I would pick Mining. Overall, you get more things to sell by mining... ores, stones, gems, motes of earth and fire, arcane crystals... just to name a few.