I like both priest heal specs better than holy pally. If you go pally, you get only two specs since you don’t tank. You mentioned holy priest needs to dps, but it does not and you don’t have to. None of the dps skills affect healing like disc. You could make some level 110 trial characters to see if you like melee or ranged.
Although you want to heal, you will spend a lot of time on the dps spec in the open world. The shadow priest rework is looking incredibly cool and they are happy on the priest forum. Ret pally is basic and spriest is more involved.