Tricky. They're fairly balanced.
Disc definitely has the upper hand on Infest handling, and on providing more soaking for P3 Vile Spirits with Pain Suppression.
Holy wins on overall heal capability, Body&Soul can be a fightbreaker for Defile, and Guardian Spirit is bordering on cheating as always. :P
Kinda your pick. Our only kill so far was with 2 Resto Shamans + 1 Holy Priest, many tries were with a Disc Priest however. DPriest makes the whole fight feel more controlled, HPriest makes the uncontrolled parts feel more recoverable.
-- (The Death of Rats, Terry Pratchett, Soul Music)