Higher chance for instant kill: Black Manacles vs. Creeping Death?

I am leveling the Necro hero class at the moment and I wonder with of the mentioned weapons (Black Manacles or Creeping Death?) has a overall higher chance to kill an enemy by it’s effect.
Manacle is a higher inital chance (20%) to insta kill an enemy, but Death marks two enemies and the marks have the chance to stay several turns. So which one have the overall higher chance?

Devour is stronger than deathmark

You can cleanse deathmark but not devour