While levelling just go for the highest spell power 2H or MH/OH combo you can find. As a shaman you will never use a 2H in end game in any spec. You do want to get out of the habit of Meleeing though as increasingly you will hit like a wet noodle and it is not worth it.

At 80:
Advantages of MH/OH combo is more spell power. In PvP you will almost certainly use a shield, in PvE it matters less as the only shield enchant that increases dps is a poor dps boost - if you dont raid the chances are you will end up not using a shield as there are no decent non raid PvE shields for an elemental shaman.