I've found my druid team to be amazing at the leveling.
Dumping Rejuvenation and Regrowth from four toons onto one, basically makes it immune to mob damage.
They're all boomkin hybrids, with a touch of resto.
They easily leveled a lot faster then the shaman team.

The shaman's seem to be much more versatile.
In particular, you can destroy casters with 4x Grounding Totems, and 4x Earth Shock.
They don't quite have the sustained dps of the druids, but they have very effective burst - which is very strong in pvp.
In arena, 4x shaman's plus someone else playing the healer... is far and away the most successful (to date) boxing combo.

In Moonkin form, they have similar armor values to mail wearing shammies with shields.
Either class could have a strong healer spec, and until heroics the dps spec x4 can very effectively heal.
As far as pvp goes... Tremor totem is the one thing which pushes the shaman's very far ahead of other boxing combinations.