Either works, its a preference.

The Priest automatically dies early in arena.
At least until they have a lot of gearing done.
The Spirit of Redemption absolutely rocks, free healing that cannot be interrupted directly (just Cyclone a DK, as far as I know).

The Pally has the bubble mechanic, making them a poor first target for many compositions.
But undergeared, they will die very quickly too.

With gearing, I'm sure both will rock.
My Pally is 2.8k res, almost full honor gear now.
That's over 80% melee mitigation, if you add armor + resiliency; she can take a beating, but smart hordies in BG's do the chain fear/cyclone/horror, etc.