Disc or Holy: Which is better?

Okay, so I was playing in a random group in a World of Warcraft dungeon (I can't remember which one, though) and as many PUGs do, this one broke down into an argument about which spec is better. The topic of discussion was Discipline specc'd priests and Holy specc'd priests and which ones were better for healing dungeons and raids. Our priest, who was specc'd Discipline, of course argued for the cause of Discipline heals. Awesome mana regen rate and heals were great he argued. Another member of our team who had a level 85 priest on another server (don't we all, now), countered that Blizzard had nerf'd the Penance spell hard in the last patch and now Holy was the L337 spec. I didn't get involved in the argument at the time, because I simply just wanted to finish the dungeon and get credit for it without having to wait in the queue line again and start all over. I do have a level 32 priest, and I realize that it doesn't really matter until end game what I think about the spec, but I simply do like the Discipline class better than Holy. Penance still works really well in my opinion and the mana regen is definitely awesome. Plus I can solo through my quests instead of feeling really squishy as I did in Holy form. So I think it's a good levelling build, but is it good for end-game? I don't know, I have heard of raids where they didn't want a priest healer unless they were specc'd Disc. I have also heard of raids who didn't want a priest healer at all, so I have heard all sides of the argument. It may just be a player preference and how good a person is at playing their class. Let me know what you think in the comments section!