Very confused priest for legion (dual spec question)

I plan on maining holy, normal/heroic raids and mythic dungeons. I really want to also be able to play as shadow if healers aren't needed in my guild. I'm confused as to how I'm supposed to level because of the weapons. Do I just level as holy? Will I nerf myself if I start out as shadow? Very confused as to what I'm supposed to do...would love any input. Thanks