im in both situations... i do raid healing when we have a better suited healer. otherwise i go on the main tank. but im still aoe healing cuz i want to.
cuz right now we are running 10man naxx with me and a paladin healer. the pally healer has less stats than i do, so i go on the main tank, in situations where i run with other guilds or 25man (not very often) i usually get sent on aoe healing.