Disc vs Holy

Having done most of the icc bosses some times gona put my gut feeling on what spec i think the raid would benefit most.
I'm talking about experienced players in the spec cuz i'd be proly mostly better in holy as this is what i'm used too. And provided the neccecary tank healers / raid healers are present to fill in the other roles.

Lord Marrowgar - HOLY
Lady Deathwhisper - DISC
Gunship Battle - DISC
Deathbringer Saurfang - DISC
Festergut - HOLY
Rotface - HOLY
Professor Putricide - HOLY
Blood Princes - DISC
Blood-Queen Lana'thel - HOLY
Valithria Dreamwalker - HOLY
Sindragosa - DISC
The Lich King - DISC ?

Since i prefer disc ofc i would be happy beeing disc in each fight But i do agree to a certain extend and it also depends on the other healers ofc. If you already have a shaman and druid healer for example then disc all the way except dreamwalker. Dreamwalker cant be done (at least not with me) in Disc.

Lord Marrowgar - was doable in disc i think there's no needed spec here.
Lady Deathwisper - DISC
Gunship - DISC
Saurfang - DISC
Festergut - HOLY idd. Did it as disc but it was really really awfull
Rotface - Holy would be easier but def doable in disc
Professor Putricide - DISC imho in the raids i've been in it was really usefull to have the survivability that tanks have with a disc priest here
Blood Princes - DISC idd
Blood-Queen HOLY, was pretty difficult in disc. But i saved my devine hymn thingy until the second fear and that saved a lot of people then.
Sindragosa
Valithria Dreamwalker - HOLY YES HOLY, It is not doable in disc (at least not with me as a disc healer )
Sindragosa - DISC , I can spam shield and that saves a lot of dmg... even on the raid. You can use shields here as raid healing
The Lich King - No idea