What makes you choose a spec? The playstyle? The feel or aesthetics of it? Or are we slaves to throughput?

Usually this is an issue for classes with multiple DPS specs, such as warlocks or rogues. But priests are unique in that we have two healing specs. So the considerations are just as valid for us.

On my warrior, I use Arms as my DPS spec. I like the feel of it. My backstory for my warrior is that she's very proud and very skilled. She's precise as well as powerful. Arms works for her in a way that Fury wouldn't. But... my warrior is an alt, and the DPS spec is her off-spec. I don't feel pressure to top meters, since it's just for fun, anyway. My priest is very different.

In Wrath, I was Discipline. Why not? Mana was effectively unlimited, so I could spam bubbles all day long. Damage prevention was very important because damage in Wrath was extremely spiky, so Discipline's damage prevention was important. It was tedious, admittedly. When your healing charts are 30% shield, 30% glyph of shield, 20% Aegis, and 20% Prayer of Mending, fight after fight, well, let's just say it's not a compelling playstyle.

The changes to healing in Cataclysm were terrific as far as I was concerned. The re-emphasis on mana management, on making sound decisions, on anticipation and thoughtfulness, was much appreciated. Those are things I loved about being a healer, things you don't find in other roles, and I've enjoyed healing in Cataclysm. The talents/abilities shakeup went a long way, too.

But which spec to choose? I dual-talented Holy and Discipline and played with both to try and find out.

After my various five-man experiences, I felt more comfortable with Holy. Holy had more mana issues than Discipline, but I felt like the group-healing throughput of Holy was much better. Sure, part of that was probably people being stupid and standing in stuff-- this was pre-nerf heroics, mind you-- but I nevertheless felt that Holy better fit my needs.

For tier 11, I was primarily Holy. I would do Discipline on occasion for specific fights, but on the whole I was Holy. And I was able to do well as Holy. Not world class by any means, but well. Partly, I was playing opposite a Discipline priest, and we didn't fancy tripping over each other's bubbles.

Tier 12 that changed. For one thing, the other Disc priest had to leave for family reasons. Before he did, though, I watched him pull further and further away from me on the charts. Was it just gear? He did have better gear than me, but not that much. Was it the fights? Discipline was clearly superior on Baleroc (except for not having Guardian Spirit), but I didn't see why it should be better on other fights.

So I played around with it, going Disc a couple times, and yes indeed-- my numbers improved.

So: is Disc a better spec at this point? Or am I simply a better Disc player than I am a Holy player?

Player skill and interaction with a spec can't be dismissed. All the same, I can think of a couple reasons why Disc has pulled ahead of Holy.
1) Unity of stats. Holy relies heavily on spirit for regen. However, Spirit provides no throughput on its own. It allows higher throughput by letting you be less efficient. In contrast, Discipline gets regen primarily from intellect and tolls based on percentages of your maximum mana. Intellect, however, *is* a throughput stat, as well as providing longevity. Intellect is the best stat for everyone, yes. But because Discipline's intellect advantages make Spirit somewhat redundant, Disc can afford more throughput stats.
Okay, sure... but I use the same gear for both specs. So we can toss out that argument as it applies to my case.
More simply...
2) More mana. To put it simply, Discipline has much more mana available to play with. More mana is always better. Holy's regen is mostly passive, so you don't think about it, which is nice. But macroing and smart play take some of the management out of Discipline's regen model.
3) Absorption vice HoTs. I like Echo of Light. It's worth 10-12% of my healing when I'm holy. I like Divine Aegis more. It's worth ~25% of my "healing" as Discipline. Yes, I know that the number is distorted because a good chunk of my Aegis are automatics from Prayer of Healing, and when I'm Holy that healing value is made up by Chakra making the heal bigger. It still strikes me that, once again, absorption trumps HoT effects. Shield and Aegis are functional equivalents of Renew and Echo of Light. Guess which ones are better?
Okay, fair enough, Discipline has a better model. But why did it take until t12 for this to come out? Why was this less significant t11 than now? Does Discipline simply scale better? If that is the case, we have every reason to believe Discipline will be vastly superior to Holy come t13.

I don't know for sure. Holy does have some utility that's very nice. I go Holy on H. Shannox because Body and Soul is so good for getting resets. Guardian Spirit sure is good for countering Decimation Blade (although now that it's only two attacks instead of three it's not as important). But for throughput and style, Discipline seems to suit me.

Who knows?