Currently playing with resto druid for my shammy team and resto shammy for my pally team.

Both of them have nice utility and reasonable mobility compared to the other healers. I chose the druid for the shams because having a tank is nice for PvE stuff and I love instant roots for immobilising ranged targets to nuke.

I like the shammy for the melee team, because chain heal is great on the bunched melee and frost shock is a great snare for the pallys. The sham also brings the haste totem for the melee, which is the best melee dps buff that a healer can bring I guess.

If you are running a mixed group, why don't you consider a holy priest? Can't beat these guys for easy multibox group healing IMO with circle of healing, prayer of healing, prayer of mending and spammable HoT.