Nothing against a priest healer.
They provide strong single and aoe heals.
They add a strong buff to the group.
Discipline adds extra mitigation to a team.
Holy adds extra healing power.

A resto shaman is also a strong healer.
Earth Shield on the tank makes a lot of encounters much easier.
Totems are a configurable buff, adding more as you have fewer shammies in the mix.
Riptide gives a HoT, but compared to a priest, mobile healing is definitely weaker.
Chain Heal and Healing Stream Totem are strong group heal options.
Mana Spring Totem, Water Shield and Mana Tide are great mana regeneration options.
The toon is much less squishy then a priest.
And can wear healer cloth, leather and mail, instead of only cloth.


I wouldn't necessarily say, one over the other, as an automatic decision for healing.
And if I had a Priest and a Shammy in the same team, one had to heal and the other dps...
An Elemental is further ahead of Shadow, then either class is ahead of the other healing.