Holy or Shadow?

I have played my priest as both holy and shadow spec. It gets pricey trying to constantly tweak your talents to suit your needs. To be brutally honest, though shadow is the preferred way to go for PvP and leveling, I find Holy with a good amount of Discipline is both easy to level and great to group with from beginning to end.

As a Horde priest, you have many opportunities to start doing instances from an early level. You get better loot and can finish more quests. Sure, shadow priests can still heal pretty well in early instances, but they lack the healing capability to carry a difficult instance group. I have also solo-leveled easily by using a mix of attacks, instant casts, and a smattering of high-powered heals. I have also taken down opponents in PvP 2 lvls higher than me while out soloing in hotspot PvP areas like Tanaris and Stranglethorn Vale.

The downfall of being a Holy Priest (and trying to solo) is you tend to burn a bit more mana while trying to level. I found myself having to regen more frequently, and needing to use big damage spells like Holy Fire and Mind Blast to pull mobs. I also ended up having to kite mobs while casting instant spells like Shadow Word: Pain and Renew in order to conserve mana while trying to survive.

So here's the verdict: if you tend to group with others (even if it's one person), go Holy. You can pull back, conserve mana, and support your party. If you like to solo and find yourself in a lot of PvP situations, Shadow may be the way to go. You can still do instances, but you won't be able to heal to your full potential.