It is possible with both.
Disc will even heal while doing so and Holy has Smite Chakra which also buffs shadow spells like mind sear.
There was even a boss in HoF where Holy did more damage than Shadow with the right gear.
Also, some time ago Disc was the better damage spec for low level dungeons since Shadow went oom too fast.
For RP Disc is most likely the way to go as it is more consistent if you want to use only holy damage spells for damage and it gets Penance early enough.