Having played an end game healer for 5+ years, on two different MMOs, I have little experience on the subject of healing. A healer is not a healer is not a healer is not a healer. The mechanics of a healing class can favor raid or a single target healing depending on the spec of the healer. Paladin healing is strongest on a single target, but how many raid bosses let you stand still? Druid healing is awesome but those hots take some time to preform. Priests and Shamans have a good mix of group and single target healing, awesome smart healing spells, with some damage reduction. If I were to pick a better healer between the two Priests wins out over Shamans by a hair. But the Shaman has more gear choices available to them and could gear up much much faster.
Right now this is the healing assignments in raids...
-Paladin/Disc Priest on MT
-Holly priest/Shaman/Druid on OT and Raid healing