It's a tough choice.
Who is harder to kill, when you focus on them.
A disc priest or a resto druid, who is pvp geared.
From a playing them point of view.
They're both very mobile healers.
The druid kind of needs to pre-hot the team.
But you can do that with a castsequence.
Kind of like....
/castsequence [target=5] ,,,,,,,,,,,,,Rejuv
/castsequence [target=4] ,,,,,,,,,Rejuv
Etc.
Do that with each of your hots and roll them.
The priest has instant Holy Nova, which is more healing then you normally need.
And that is totally mindless for battlegrounds.
Not sure either healer will work in 5's, in arena, or which of the two would be better.
When someone is focused, the priest has Pain Suppression, PW: Shield and Flash Heal/Penance.
I'd say the priest is better at reaction/twitch healing, so I went with them.
But the resto is no slouch; I've had times when its hard to drop one tree with a team.