It depends on what you want.
If you want powerful group heals, a quick single target, excellent buffs, and often relying on group heals instead of single target healing, go with Priest. If you want excellent survivability with those, P/K is your best option.
If you want lots of Heal Over Times (D/S) , a raid-wide instant group heal, great cleanses, and mostly single target heals (Though some combos have alot of group heal ability), go Druid.
Neither is really better or worse in their healing ability, it's all about how they are played. A well played Druid and a well played Priest will heal comparably. However, Druid is much harder to play correctly than Priest, on account of all the extra things they have to worry about (Cleanses, more single target, lots more skills to worry about levelling and using.)