I feel like they should be equally strong, like they are the opposite sides of the same coin, but in the game it looks like Holy classes pretty much make shadow their bitches.
Paladins have abilities like Turn Evil, Wake of Ashes, Shadow Resistance Aura that give them extra power by having their way with shadow themed creatures. Priests have Shackle Undead, etc.
Meanwhile, DKs and Locks have no such power that helps them against holy/light creatures.