Tough call.

They both bring strong buffs, can both rez.
Both heal fine single and aoe.

Druids are HoT based, so you'd like to keep the HoT's rolling and precast.
They are impacted less by movement encounters since HoT's are instant cast except for Regrowth.
Nature's Swiftness, Innervate and Battle Rez are extra for them.

Priests (Disc) can bring a ton of mitigation to the mix.
If you're not using a click castsequence to automatically apply HoT's, they're probably easier to box.
PW: Shield, Holy Nova, Pain Suppression, Power Infusion are some of their extra's.