For questing and leveling specifically, I really strongly prefer disc. Here's why:
- +15% int gives a free spellpower bonus that increases dps
- Evangelism increases damage and reduces cost of smite, which is the primary holy damage spell
- Evangelism also increases penance damage and reduces cost, and penance is a big boost to dps
- Stronger shields mean fewer interruptions, and atonement can reduce the need to heal yourself should they get through the shields (which normal grinding mobs probably won't. questing is very rarely about "live or die," it's about how fast you can get it done)
- Power Infusion makes elites drop quicker and is fun to throw around if you're questing with a caster buddy
Now, in terms of dungeons, and especially non-heroics, I know this is controversial, but I personally can't imagine any reason to go holy over disc. Disc is truly amazing and fun in dungeons. Pre-shielding people requires timing and is a little repetitive, but with disc unlike with holy you're always engaged. If there's little damage out there, then you should be doing more dps (and you'll do much more than holy), and unlike holy (with the exception of surge of light), your dps benefits you in multiple ways down the road. I can pull out about 10% of the group's dps in heroic Forge of Souls, about 12% in Pit of Saron, and more than that in easier ones like Violet Hold, but I'm stuck to maybe 7% or even less in Halls of Reflection, and it's kinda fun to see how or where I can improve on that. Plus, I feel like I'm contributing to the group in many more ways than just simply making sure they don't die. Mobs die quicker, I need to heal for less time, and I have a lot more fun doing it.
Now, I'm not hating on holy. It's nice. It's my secondary spec, it works, and for more than just AoE healing like everyone seems to think. But as to the OP's request, which is (non-heroic) dungeons and questing in a healing spec, there's no choice in my opinion. Disc. 100%.