Short answer: Do both.

Got duel spec? Okay, good. Now make one spec Holy and the other Discipline. You can get your 50g back doing your random daily later. (Need to have a DPS spec for switching roles because you have too many healers? I’d suggest switching roles per raid instead of per fight; it’ll simplify the dilemma.) Now I’ll go over the details why.

There are several bosses where you can play whatever floats your boat. Some say it’s great to have the damage reduction and bubbles in a raid for all fights, others say the impact Holy priests make on the charts is astronomical. If you want to be a winnar though, you’ll need Discipline for Saurfang and Holy for Festergut (some call him Sporeface.)

For Saurfang: Your bubbles are going to slow down how fast he gets runic power, and therefore how quickly he starts to mark people into a pit of utter doom. You’ll be amazed at the difference you make in this fight when you’re Discipline; no other healer comes close. (They might think they do if they’re chart whores though. )

For Festergut: Raid damage EVERYWHAR. Holy has an advantage here, letting you run around and CoH, not to mention the Serendipity build-up for your huge PoH spells. Plus, I’m a Holy fan, so… of course I’m going to recommend it for something. :p

Of course, if you don’t have dual spec–or you’re just unwilling to spec Holy and Discipline because the concept seems asinine (it used to seem stupid to me, too)–then you can play whatever fits comfortably with your playstyle. I personally stand by my theory that Discipline is more useful and Holy is more fun, but this is an opinion that can be supported or diverted with endless amounts of subjective numbers and theorycraft. In the end, you should do what’s right for you, and what’s right for your raid based on the other healer you’re paired with.