They are both step-by-step instructions with maps on how to level to 70 in less than 10 days.

10-14: Wand Specialization 5/5
15-16: Improved Power Word: Fortitude 2/2
17-19: Improved Power World: Shield 3/3
20: Inner Focus 1/1
21-23: Meditation 3/3
24: Martyrdom 1/2
25-29: Mental Agility 5/5
30: Divine Spirit 1/1
31-32: Improved Divine Spirit 2/2
33: Make sure you get Force of Will. I don’t know if I’d go passed Force of will. You may want to consider building up another talent tree.

Holy Priest

There are a few priest spells that cause some damage, such as, Smite and Holy fire, but you will still be slow to level. It is possible to get by with a Holy Spec but usually you want a friend to help you quest and level when you’re not in instance groups.

10-14: Holy Specialization 5/5
15-19: Divine Fury 5/5
20: Holy Nova 1/1
21-22: Healing Focus 2/2
23-24: Blessed Recovery 2/3
25-26: Searing Light 2/2
2728: Holy Reach 2/2

Basically your priest damage spells are Smite, Holy Fire, and Holy Nova. When mobs are low on health or you wish to preserve mana use your wand.

Shadow Priest

A shadow priest is by far considered the greatest spec'd priest for PvP and PvE. You can solo mobs with little or no downtime very effectively with this build using it’s various taps and DoT’s while fearing and wanding mobs down to nothing. Spirit tap provides excellent mana regen by doubling your spirit after exp yielding kills, making it harder for you to run low on mana while pwning with your priest.

10-14: Spirit Tap 5/5
15-16: Improved Shadow Word: Pain 2/2
17-19: Blackout 3/5
20: Mind Flay 1/1
21-24: Improved Mind Blast 4/5
25-26: Shadow Reach 2/2
27: Improved Mind Blast 5/5
28-29: Shadow Weaving 2/5
30: Vampiric Embrace 1/1
31-32: Improved Vampric Embrace 2/2
33-34: Focused Mind 2/3
35: Darkness 1/5

Continue filling out Darkness, Shadow Weaving, Focused Mind. Get Shadowform when available, then work your way toward Vampiric Touch which is well worth it. Simple strategy is: Mind Blast, Shadow Word: Pain, Mind Blast, Wand or Mind Blast, Shadow Word: Pain, Mind Blast, Fear, Mind Flay, Mind Blast, Mind Flay

Summary

You can either be the greatest healer in the game or one of the best PvP classes. It depends what you want to do in the end game with your priest. As far as leveling, you won’t have any problems going shadow priest. It’s not as fast as a hunter, but definitely is up there among some of the fastest classes.