I would take a disc priest over a holy priest anyday and I have both. The way I see it is "Will CoH prevent your group from wiping?". I spend most of my time healing my tank. The dps takes splash damage, random charges, even some chain lightning when I forget to turn the mobs away from my group in heroics, but nothing that will one-shot them. Penance(51st talent in the disc tree) is the main heal that you would be using if you're disc. It heals your target 3 times while channeling for 2 seconds and the first heal is instant. All three of those heals can proc Divine Aegis(Should spec for it) when you crit on your heals, which absorbs more damage. Grace is a buff that your targets get when you heal them with Flash Heal, Greater Heal and Penance, that increases your priests heal on target by 3% and stacks 3 times. Each tick of Penance applies the buff, so one uninterrupted penance and you're at the full stack. Renewed Hope is another goodie, extra 4% chance to crit on FH, GH, and Penance on targets affected by weakened soul which is a debuff from Power Word: Shield. 2/2 in Renewed Hope also reduces damage to the party/raid by 3% for 20 seconds when you cast PW: S. The weakened soul debuff is 15 seconds, so you could have 100% uptime on the 3% reduced damage and 4% extra crit heals on your tank. The down side is your priest absolute sucks on the healing meters, the upside is you don't take alot of spike damage.

I have healed while I multiboxed and single box on a holy priest, it required more attention than while I'm healing on my disc priest. My holy priest is in Naxx 10 gear and my disc priest is in heroics, emblem, T7 chest/gloves and a Naxx 10 ring. Another thing I like about disc is if you get a big enough mana pull, you don't worry about mana anymore. When PW: S is completely absorbed, you get 2.5%(3/3 in rapture) of your total mana back. For my disc priest, my total mana is 19,423. It's about 769 mana each PW: S, 10% reduction in mana cost from Mental agility, and 15% from Soul Warding. Rapture returns 485 mana when PW: S is comsumed. With a holy priest, I'd have to watch for procs on flash heal, stay out of the 5 second rule, and keep holy consentration up for mana regen. Give it a try, you might like it.