Yes I forgot about the mastery buff from shaman which can be Huge for DKs in both pvp and pve. In full PVP gear you can push well over 40% mastery with the shaman's buff which is going to significantly affect howling blast, frost fever, and frost strike, but not obliterate. With the druid you get mark of the wild of course which will give them more strength, affecting all abilities by a small amount.

Resto shaman may have more difficulty surviving as your main healer but then again you have a lot peels with 4x death grips, asphyxiate, etc. But the shaman will need to be more stationary a lot of the time. Druids as I mentioned can move about and get out of tight situations themselves without as much peeling. Both, I think, have the capability of keeping all four alive in tight situations, resto shaman with ascendance, healing rain, healing tide totem, and especially spirit link. But I still feel like tree form has a slight advantage, and if you have a chance to go offensive you can contribute some CC to the group, which shaman don't have much of.

I'm talking specifically about PVP here though.