I would probably go with a Ret Paladin (DPS spec) or Prot (if you're doing dungeons) and a Disc Priest.

Disc because your healing is decent, if you take into account your mitigation abilities it is very comparable to Holy.

As a Disc Priest, you get Penance which is your best DPS ability.
Eventually, Smite will heal for as much as the damage it deals (talented).
PW: Shield will prevent a good amount of damage too.

All my priests (six of them) are Disc.
But Holy does work too, if you solely want to heal and won't PvP or DPS much.