Assuming you are 2-healing the 10 man like you should.

How exactly does DA and PW:S do nothing on P1? You can keep both of them on both tanks almost the entire time and, if you time your rapture correctly, have full mana by the time you get to the sons of flame. DA from a GH is also the same size as, if not slightly larger than, a PW:S. This essentially makes a GH heal for >100K when you include the DA, which will actually always be used because the tanks have a dot on them even when the other one has taunted off them.

Just stating it doesn't make it true, you need to say why.

From a raid-healing perspective that is =)

The P1 raid healing is very light in comparison to the tank healing. You only really have to heal up after traps or throw one heal out on someone who took a wrath before the next trap is due. The raid will be at 100% almost all of the time.

Since people are spread for wrath, holy really doesn't raid heal traps much better than disc. If you can get all your ranged to stand near each other, precast a prayer of healing on traps. PW:S can be useful to spring a trap after a wrath. If someone takes a wrath, you don't need to heal them up just throw a quick shield on them before someone hits the trap. CoH would be a nice addition for healing up after a trap, mainly just because it's not limited by group so positioning isn't so much of an issue, but really the traps are timed so far apart and there's so little P1 raid damage in normal mode that you have plenty of time to get people topped off.

Absorbs do little to counter traps, as does Barrier. Have you tried 2-heal this fight? I wouldn't say the raid is topped off most of the time in P1.

Also there's the issue of AA/A not working on Rag's huge hitbox.

I'm not saying it can't be done as disc, especially on normal, but holy is better suited for the fight in question, even the world's top priest (Jhazrun from Paragon) admits it though he has historically preferred disc.

Is Holy really better for Heroic Ragnaros on 10-man as well? Honest question.

I've been Disc all the way this expansion, so I'm not challenging the statement; I'd just like to be sure of the reasoning. We should be starting on H Rag next week, so If Holy really is much better, I'd like to get used to a Holy spec before that happens. Usually I'd just say "Screw it," and stay Disc anyway, but given the difficulty of the fight -- still only 33 guilds in the world who've downed him currently -- I figure we'll need any edge we can get.

We 2-heal normal with me as Disc and a Resto Druid as my healing partner; currently he's on tanks and I'm on raid for P1 (because he wanted to test out his 4-piece the week we started 2-healing, I think, and then we just never switched), but that'll probably change on heroic. In any case, keeping the raid topped off in P1 hasn't been much of an issue. Barrier is nice for P2 seed explosions (though according to my v. vague understanding, with the strategy Paragon used for H Rag, you basically just run away from the seeds as a group, so Barrier wouldn't be necessary?), and the healing required in P3 is minimal.

I know the fight is extremely different on Heroic, so I'm definitely willing to consider the idea that Holy is the way to go.

We haven't yet reached him on Hc yet (5/7 for now), but judging by the fact that 25 HC world first was done with 3 healers (Paladin, Druid and Holy priest) I would say yes its better than disc.

However, in 10 man where you take one healer out, priests (of any spec) can't compete with druids for the spot, so its a rather pointless discussion ^_^

So if you want the best setup possible (and otherwise you won't kill him, really, just look at the # of guilds are at 6/7 and at 7/7 -_-), you would go with a paladin and a druid.

We don't have a Holy Pally (our off-healer is a Shaman), so yeah, not exactly a pointless discussion for me. I don't expect that we'll get Ragnaros down SOON, but I do expect to get him down eventually, even if it's at the end of this tier, and by then I would hope there'll be more healing setups for 10-man kills than just Resto Druid/Holy Pally. I'm willing to respec for this fight, but not to level up my 70 Pally and relearn how to heal on her.

The current top-ranking priest on H Rag (25) is actually Disc -- his guild's healing comp was Resto Druid, Disc Priest, Resto Sham(!!) -- so obviously different comps are viable. I was just wondering if there were any specific mechanics on H Rag that made Holy substantially better for the fight than Disc on 10-man; sadly, there are no 10-man H Rag parses up on WoL to inspect.

If you want to see how a fight can be handled by a Dpriest in 10man:
http://www.youtube.com/watch?v=eP-CHm-7qNs

We tried different setups, and concluded that any would work (btw, there a rumors of paragon having soloheal the boss with a Rdrood) exept Rsham + Dpriest
P1 : It's just a joke. Even with a shaman on tanks, and a disc on raid, we were at 90+% mana at the beginning of the transition
P2 : THIS is much more interresting. Your setup need to be able to keep everybody alive, with enouch mana remaining (i'd say 60% without regen available is ok). And this part is not about healers, but DPS killing the adds fast. True, a Setup with a Rdrood (huge hots) and Dpriest (DA) was really efficient. But if the adds are killed before reaching melee, this souldn't be an issue.
P3 : Regen phase
P4 : Won't talk about it, since i'm not there yet :)

BTW: I'd say that in 10 man, Dpriest > Hpriest, due to their DA and PW:B in p2, and a better assist on tanks in p4.