Simple. Which will be better in 3s as of 5.1: Mage, Lock, Shaman or Mage, Lock, Druid and why? Lock would most likely be playing Demo, Mage playing frost.
If you have a druid and a shaman both lining up, take the better player. With that said, I personally would rather play MLD - I think the advantages of Rdruids this season outweigh the traditional superiority of the MLS format.