Nostalrius Begins - Quality wow vanilla realm (1.12)

I started leveling a priest with a friend of mine who is gonna play prot paladin. The idea is to be able to easily form groups for elite quests and dungeons, but beside this we really like the roles of tank and heal. He is going for consencration first and then he will start putting talents into prot.

Now, I am thinking to go for Holy Nova first. While leveling we will try to aoe grind. He can normaly do that as prot, but I think that my nova will add some more damage and also some healing too. Thinking of start like this:

http://db.vanillagaming.org/?talent#bZbht

Now my question is, should I continue into holy for Improved Healing/Spiritual Guidance:

http://db.vanillagaming.org/?talent#bZbxtcs0Vx

or should I start investing into Discipline:

http://db.vanillagaming.org/?talent#bVRhsV0oZbht

btw, I dont wanna get spirit tap I dont like the gamestyle it promoting, trying to get a killing blow and then go to next mob quickly to benefit from spiritual guidance...

So what do you think is best for duo leveling/dungeons with a tankadin?

Lastly, just a general priest question, not relevant with leveling: will this work for level 60 dungeon healing? http://db.vanillagaming.org/?talent#bVRhsVVoZbxtcs0V Is 10% more heal better than 10% more mana?

Holy nova is too expensive, just have him judge crusader for the extra holy damage and smite the lowest health target. You will see holy nova will make you drink after every battle. I loved grouping with pallies for those high damage smites and holy fires.