Both Discipline and Holy are perfectly capable of healing any five man heroic; you will generally find, however, that Discipline is the more efficient of the two -- especially at earlier levels of gear -- thanks to the talent Rapture.
As far as raids go, Discipline and Holy are the two worst tank healers. Holy Paladins, Resto Shaman and Resto Druids are all superior for tank healing. Unfortunately, the misconception that Discipline Priests are tank healers has become widespread as many players continue to cleave to the early Wrath attempts on Blizzard's part to make Discipline a viable tank healer.
Simply put, Discipline lacks the throughput to be a strong tank healer except in the easiest of content, and Holy lacks the efficiency to be a strong tank healer except in the easiest of content.
(You will, however, occasionally see one or both specs used as a secondary tank healer; heroic Lich King, notably, is one place where an alternative spec for Holy -- using Empowered Healing -- becomes desirable due to incredible tank damage being dealt; a Holy priest on that fight tends to tank heal during Phases 1 and 2, while raid healing during Phases 1.5, 2.5 and in Frostmourne)
Currently Discipline is an incredibly strong raid "healer". Bubble botting, aka Shield spamming, has become very powerful due to the ICC raidwide buff.
In five mans with little party-wide damage, you'll be focusing primarily on the tank -- keeping PW:Shield up as much as possible, using Prayer of Mending on cooldown, Penance as your go-to heavy burst heal and Flash as your filler. In five mans with noticeable party-wide damage, you'll want to keep PW:Shield on everyone as much as possible; the other spell uses remain the same. Prayer of Healing and Holy Nova can be used for heavy and light (respectively) damage beyond what your Shields can bear; try to use Prayer of Healing in conjunction with the Borrowed Time buff. You can also pre-Renew people in anticipation of damage (only advisable when you've Shielded them already and do not need to cast another spell like Penance/Flash/Binding Heal.)
Holy remains strong in its fill-in-the-gap healing style, offering Druidesque healing during aura-damage fights like BQL while retaining strong burst capabilities, which are excellent during fights like Professor Putricide.
In five mans with little part-wide damage, you'll want to keep Renew on the tank, cast Prayer of Mending on cooldown and use Flash Heal as your main direct heal. Greater Heal should be used sparingly and preferably when you have 3 stacks of Serendipity. When party-wide damage is present, keep Renew on everyone and use Circle of Healing as needed. Prayer of Healing, like Greater Heal, is best used with 3 sstacks of Serendipity.
Discipline and Holy are both raid healers and should rarely be given the task of tank healing.
For more information, check out my blog, Priest It Forward. It offers run downs on both Discipline and Holy, including descriptions of their current optimal healing styles, gemming choices, glyph options, builds and pre-raiding gear.