I would love to see someone do an in-depth talent analysis for Priests (well all classes really). Really going into the nuts and bolts into what talents are must haves. What talents are nice to have and what are trash. Even giving some insight into situational places where having something might be good. Most YouTube guides I've seen have shown a talent build but haven't said why.
I know back in the day there used to be things like this. But it would be good to have an updated version taking into consideration all the things we know now.
So something like:
Tier 1 Discipline
Unbreakable Will: This is a great talent for PVP as it gives you a much needed up to 15% resist to Stun, Fear and Silence. It should be chosen cautiously in a PVE setting, especially during phase 1 when you are regularly farming Onyxia as a fear resist could cause her to turn to you in the raid and breathe, wiping the raid. Depending on your tanks this may make wand specialisation a more favourable PVE talent (this should be less of an issue for Alliance players with Fear Wards on their tanks).
(Other than Onyxia I can't think of any other time that resistance to Stun/Fear/Silence might be useful when raiding. I do remember well the banshee's silencing in UD Strat).
Wand Specialisation: Generally taken as part of any good levelling build to increase your wand damage by up to 25%. During phase 1 especially this could still be a good option if you are needing to a lot of questing/farming solo and you have been having issues with fear resists whilst downing Onyxia.
Verdict: For PVP Unbreakable Will is the clear winner. Neither talent is a must have for PVE healing so make your own choice here depending on your faction / how much solo grinding you are doing.