Every time I do a raid with another priest, I come to the same conclusion.
A raid leader would either need to be nuts, or really clever, to take a disc priest over a holy priest.

Here's the first thing anyone will see when they look at our performance in Recount. Here "S" is a CoH holy priest with about the same gear level as me (with a totally different stat profile of course). "B" is another CoH priest, not only lower gear quality, but out to lunch, so pay no attention. All charts are from a partial run of TK with a decent quality pug. And while it's instructive, I wouldn't draw too many conclusions from this data set!
(Oh and if you're not familiar with Recount, it's basically
the add-on to track battle stats. Bars are color-coded by class, so the white bars are the priests, green are the hunters, etc. And of course the window title indicates what graph you're looking at. So in the graph at left, it's showing that "S" did 2.09m in total healing, and I did 862k.)
Thanks for coming Paolo, you've been...a really nice guy. Keep practicing!
Mitigation you say? Yeah, ok, whatever, would be great to see you do more healing though. Meanwhile, as long as we're waiting for loot to be distributed, might as well have a look at some other pages in recount, just for kicks.

Oh interesting! He sure kept himself busy doing...whatever it is disc priests do. Since it wasn't healing, maybe he was...buffing everybody? Hmm. But he cast about 25% more spells than CoH man, for what that's worth, even though CoH is instant-cast, and "S" is probably spamming it most of the time. Anyway, at least it seems he wasn't afk, so I can't boot him for that.

Wait a second, I thought we had three priests. Why weren't the others doing any dispelling? We have all sorts of magic debuffs to get rid of...damage increasers, sleepers. Not sure exactly how to measure the value of this, but damn, he seems to be pretty attentive to the raid.
So how do I score this guy? Seems seriously engaged, using
all his tools, attentive to the raid. Supposedly he has some
awesome buffs, but who knows, can't see those in recount either. And he says he does a
ton of mitigation...yeah, ok, probably. His 13% haste (with
talents, plus haste procs) doesn't show up anywhere on the charts.
But one thing that
does show up is that his heals hit for less, at least 15% less. And his overall healing done is less than half of what our CoH healer does, no arguing with that.
So? Do we bring a disc healer or not?