While the typical answer to this question that Discipline Priest is better, that is not exactly the case. This question will largely depend on your raiding environment, as both Disc and Holy fulfill different healing jobs in a raid. Disc is much better at spot healing and quickly responding to individual targets taking high damage, whereas Holy greatly excels at AoE healing, quickly topping up multiple raid members. Choose the spec that best suits your raid's needs. In short: Disc for single-target healing or Holy for AoE healing.