Right now the throughput and toolbox of holy is better suited for heroics in my opinion. Guardian Spirit flat out saves wipes if you get behind on tankhealing, and your aoe toolbox is much better. Holy also has more throughput. That said, I love the disc spec and while I've just started on healing heroics, I would like to say that I've had only limited trouble with halls of origination on any boss/trash, including spike tank healing or aoe pulse healing. Those results were with good groups that CC'd on trash, and knew the fights coming in. I've really not noticed much difference in mana efficiency between holy or disc from healing normal modes, although disc will give you more mana from the passive intellect buff. If you do want to play disc, use your cd's proactively. I have far better results in using them to prevent panic situations rather than using them once you get into a panic situation. Disc does not shine when u have an insurmountable amount of dmg to recover from. It's all well and good to conserve mana, but you cant be unwilling to pull the trigger on cds, nor can you afford to heal too conservatively.
I don't buy into the thought that certain fights or types of content are impossible if you are a certain spec. If you play well enough and think through the content things will work out. I can play holy just fine, but since I am more profficient at Disc I am better on mana as disc, and I prefer healing as disc. This is new content, you're going to want to be on your game so play what allows you to best do so.
Generally regardless of class or spec, if you have a bad tank, dps, or healer you will fail in a heroic. If you refuse to CC or proceed carefully, and if you don't know what you are doing things wont work out well, and that's just what was intended. Healing is more difficult across the board, be prepared but don't panic. If you can, run with your guild, they know your abilities and they will work with you which will save you the insults, and unwarranted blame. This is current pve content and a part of new progression. It needs to be treated as such.
Hope this is helpful.