It's comparing apples and oranges.
Holypriests can do two point five million healing done in a 15 second interval. *
Restodruids cannot.
Restodruids can pump out a stable 40k HPS for the full duration of a 10 minute fight. **
Holypriests cannot.
Which is better?
( Answer: The discpriest, monk and sometimes a paladin. )
Also: "always" is a bit delusional. Best AOE healer in the dragon soul was a paladin, remember? Doesn't mean hpriests were bad though
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*) Some liberal interpretation of reality may apply, and it's a trick you don't do twice.
**) See the previous annotation. Also, extreme mastery makes hpriest quite druid-like. Don't sweat the details.