So most of my Priest exp comes from being Shadow, but I'm leveling a Priest atm and I want to heal. I'm not very familiar with Disc/Holy and haven't been since WotLK and early Cata when I would switch if a healer wasn't online to raid. Currently, I'm disc, but I'd like to know what should be chosen for healing heroics, HS, and raids?
The trend nowdays is to go disc, there are very little holy priests raiding out there compared to disc priests. Disc offers lots of absorbs which can help ease boss mechanics or even avoid them completely. Disc also offers better mana regen and the ability to heal while dpsing, making it the perfect support role. Holy is a pure healers, it can produce gigantic numbers on the screen, and it can do so both AoE and single target. The problem is Chakra mechanics prevent you from healing both ST and AoE effeciently at the same time, making a holy a problematic spec in 10m raiding. Holy also suffers more in mana-regen and relies more on spirit than disc does, and it doesn't have utilities that make it attractive as disc, which is why most guilds prefer disc in all fights pretty much, especially in 10m. However, someone who masters holy can turn it into a healing machine like no other class in the game.