i have not yet heal in Dusk Vigil
but i do found healing Paladin and warrior more comfortable than healing Dark-knight
i dont play deep in tank role, only level those 3 tank to around 40, do feel dark-knight slightly more "paper" then other 2
as for AST vs WHM, i dont feel AST weaker than WHM as long as i m not using Nocturnal Sect
however, i do find it more comfortable with WHM when doing 8 ppl dungeon