Disc has some pretty decent AoE heals, but shines at single target healing/mitigation.

I'd definitely go with Disc over Holy, but that is a preference thing.
You're not really going to go wrong with one or the other.

With Disc...
Penance is more mana efficient then anything else you have, even if you lose one of the three ticks from something beating on you.
The first tick of Penance is instant, so you can twitch heal someone more so then any other spec.

When you know you have damage incoming, you can pre-cast PW: Shield on your team.
They still get the Weakened Soul debuff, preventing additional shield casts on them.
But you (talented) basically have no cooldown between each cast of PW: Shield.

The next spell cast after PW: Shield has a massive amount of haste.
Which makes Prayer of Healing a much faster cast.
Holy Nova is still instant, and as Disc instant heals cost very little mana -- use this for topping up or AoE heals on the move.
Prayer of Mending, like Holy, is your other AoE heal.

Flash Heal is likely your 2nd heal, after Penance.
With targets at or below 50%, it heals for a very good amount.
I've gone with /castsequence reset=7 Penance, Flash Heal, Flash Heal, Flash Heal.
Works pretty good for me.

Pain Suppression, on someone who is about to take a lot of damage is half damage outright prevented.
Which can be a massive amount depending on the toon and the attack.

I've worked Power Infusion into my base big heal spell.
So I don't manage it, but it is used whenever available.

I have one Disc priest on one (mixed) team as a healer.
I don't often drag her click (dps) macro (the main "calling other buttons" macro) onto the "1" key, but its an option.
Technically I have another Disc priest as the healer on another team, but they're low so far (1x Disc / 4x Warlocks).

On another team, 5x Disc, have gone in Shadow down to Mind Flay.
Even without shadow form, Mind Flay + SW: Pain + Devouring Plague is pretty decent dps with five of them.