Disc Priests are absolutely beasts due to Spirit Shell and I'd strongly recommend getting one. We have 4 healers in my own guild and have a 5th available when needed (Disc Priest), so from experience I'd say Disc Priest or Resto Shaman. I wouldn't like to raid without both in my roster tbh. On some fights like Heroic Tsulong where Monks are good (better represented on 25 man than 10 man on WoL though), they can really shine. But overall I still think that both Disc Priest and Resto Shaman beats a Monk healer.
On progression kills, we'd always bring the combination that would benefit us the most. So our first Heroic Lei Shi kill we had a Disc Priest, Resto Druid Resto Shaman. The Druid got running Tranq from Symiosis on the Shaman, making Get Away phases much easier for instance. Imo it's not just about raw output it's also about utility and how the different healers support each other.
Anyways, Disc Priest would be my answer. We've had Disc Priest on most of our Heroic progression kills (15/16 Heroic) and it's just so much easier with a Disc Priest.