There is absolutely nothing wrong with either healing spec, problem most people have with Disc is they do not know how to play it structured enough to be good at it.
Penance, PWS/Rapture, Grace, odd creatures in healing land they are and most of the time (sorry to say) not used properly.
Holy on the other hand I find much more forgiving....
They're both pretty close tbh. Some fights Disc will be better than Holy, although, Holy is generally better in 25 than Disc, since Disc only has Prayer of Healing for AoE heals, whereas Holy can also use Holy Word: Sanctuary and Circle of Healing.
I think holy is better in 'slow' fights. Where you have to heal a lot, but not much at once. Where you have a lot aoe dmg, but noone will die within 3 secs.
Imo Disc is better in fights with a lot of burst. When you know your raid is gonna slapped by a giant tentacle, just plant the AoE bubble. A Dragonbreath is coming? Just shield the Tank.
One thing I hate as a healer is, when I lose control "Omg they are all getting that much dmg, stupid smurf Fire-loving lemmings. I can't heal that much" <- this is gonna easier to handle as a disc I think. PI and PW:B are helping a lot in situations like this.
Since you are lvl 84 that means you go for 5-man dungeons, I'd play holy. Just throw PoM and 1-2 Renew and you can go afk for a while
I got the most exp in Wrath ICC with my priest, so what I say may not be 100% accurate.
since both are viable you can pick whichever you like more and stick to it.
its not like at this point of the game you are running for worlds first. in 5mans, lfr and normal ds it really does not matter at all. i dare to say it doesnt matter much in ds hc either, however it is easier to progress on certain heroic fights with certain specs.