As a Disc Priest you can also solo a lot of old contents. Not as good as Blood DK, but you are basically the best non-tank and non-pet solo class cause of Atonement and absorb stacking mechanisms that can cheat all the mechanics that disables healing.
Finally got rid of my old sig about Pokemon after 10 years, DARN that's how long I've been on GameFAQs T_______________________________________________________T
Disc priests are situational in raids. Given the right boss fight, they're mindblowingly strong, but there are plenty of other fights where they'll lag behind. It depends on the situation. Personally, if I were trying to make a heals/dps priest, I'd go with holy, just because it's more well-rounded. You won't be as amazing on some fights, but you won't be struggling either.