So I got KSM on both in S1, and just got KSM on my priest this week.
Resto has more utility and is generally better in the M+ environment because of it. This is largely because they come with bloodlust and interrupts. The rest of their utility is good, but not really a necessity in M+ groups. If you know the dungeon, they’re also better for carrying groups simply because a lot of the dangerous stuff in SL dungeons are made a joke with interrupting properly, and if your group screws up interrupt you can easily fix that where another healer would have to burn a CD to deal with whatever was missed.
Holy Priests have more on-demand HPS, and PI is very good for boosting DPS on big pulls. But including a priest means you have to have a group with fast interrupts and a lust since they straight up just can’t do either. They can at least drop a single target stun, but that doesn’t actually stop the cast, just delays it. And if you were a shaman you could AOE stun anyway.
Also worth noting that Kyrian Discipline priest is very good, and likely to become one of the best healers in M+ as soon as the whole mikanikos soulbind is available through renown so long as nothing gets nerfed. A Discipline priest in boon does quite a bit of damage and makes the group close to invincible while doing it, and having boon’s CD reduced that much means you can be very aggressive. Add in some of the best DR in the game and they’ve got potential to be monsters.