I heard level with enhancement, then switch to elemental. I'm leveling right now, so I haven't had a chance to try elemental.
Edit: I've been using elemental since level 45ish on my shaman and it's far better, imo, than enhanced. Blizzard seems to have nerfed enhanced according to some oldschool shaman that I have spoken with, and believe me they're pissed about it. In fact, so much so, that they have quit playing their shaman completely.