Dual-wield them both.
But seriously... I'd say the Flail, simply because it has so many applications for use by poor, disadvantaged Clerics who have sod-all half-decent weapons otherwise. Everyone who can use a katana - thieves, fighters and the rest - have other, almost-as-good options as Celestial Fury. Plus, you don't have to fight through a small army of powerful characters to get the flail.