I honestly believe smite will be situational at best as we progress from 80 - 85.
I also believe that in the progression process, disk will lag behind due to poor scaling with mastery. Disc will be a good support healer, but nothing more.
Priests will be the least mana efficient class of the healers. Disc will have the lowest throughput, and their aoe isn't so great either. Even so, I'll still play disc.