Disc and Holy "numbers," whatever they may be, are both good. DPS wise, (not including trash), Disc will win out. HPS wise, who knows. I haven't played with any of the god-tier disc priests that keep posting on here, but I'm sure it's comparable.
My issue revolves around ideal and realized healing scenarios. In a raid, especially during progression, it won't be muscle-memory. It won't be mistake-free -- hardcore or not. People will stand in the bad. Melee will run out and spread the dot into ranged. Tanks will miss CD's, etc. When that happens, disc doesn't really have any "life saving" abilities. If a group of four in ranged gets smoked unexpectedly, you have to cast radiance and hope to god light's wrath is off CD and, even then, wait for the cast to complete to realize the healing. Or, you waste a few more GCD's in putting a shield up or, like many situations, you just spam shadowmend. If that same situation happens as holy, you hit the lowest with a serenity if they all won't die immediately, and sanctify the others followed by maybe a POH or 2; if they will all die due to ticking damage, screw it and rescue them with the instant sanctify. The holy toolkit just seems better implemented to me with an answer for every situation.
Last issue that doesn't get talked about a lot is when raid mechanics remove the ability to do sustained DPS on targets. Whether it be dealing with mechanics or moving from the bad, Disc is left with two spells: their shield and shadowmend, essentially. They are the only healer whose output will be suboptimal during those times. Depending on the fight, that could be exactly when you need the heals the most.
Just my thoughts on the spec.