I'd have to disagree wholeheartedly here. I find it pretty difficult to top meters as disc vs a holy unless you're getting fed innervates and such. (Talking about 25 man, in 10 man you're usually set as an (off) tank healer if there is a holy priest as well)
1) Spamming shields & DA isn't viable unless you have some very good gear, and even then you don't want to do it in heroic raids. ALSO, there are times where you want to use the holy shield rather because of body and soul (ie atramedes breaths)
2) Mana management as disc is MUCH more difficult than the passive mana 'management' as holy (where most of your spells so efficient that you don't need another way to regen mana). Hell, even the 4-set agreed with me on this; where you had to have weakened soul on a target AND penance it as opposed to just having a chakra enabled. (recently they changed this to just having to use penance)
3) "Heal sniping" (whatever you want to call it) is easier as holy because you have more instant heals in your arsenal (holy word and CoH).
Disc is there for the versatility where you can decently spot-heal, use cooldowns to prevent damage.
I like how people still try to make Disc look like it's the LOL EZ WotLK spec, whereas i feel that right now its 'easier' to raid heal as holy.