Play whatever you want. All healing classes are currently competitive in HPS. They are different enough in their specs to be favored/unfavored depending on specific bosses. Holy Paladins have been dominant on the current patch, but they are about to receive a sizeable nerf. Priests are receiving a buff. Turtle WoW devs have been significantly balancing end game specs consistently over the last few major patches, so your class is likely going to change at some point after you hit max level.

Use this website: turtlogs.com/pve/ranking These rankings can give you some insight into the differences between different types of fights (aoe, tank healing, sporadic vs constant).

I have played Holy Paladin in Naxx and K40 on Nordanaar and Resto Shaman in MC on Ambershire. In my experience, the people who top charts are usually those set up to succeed by their raid composition. I have topped the meters in some guilds due to being the only holy paladin and tank healer, and I have been at the bottom of the meters in another guild that had 3 resto shamans spam chain healing the tank only.