I can't really motivate taking Biting whip for just an extra 10% damage, when you get such an improved bonus from Draining Whip. It made me think that maybe if you add a +1 (or even +2) Penetration to the Biting Whip it would even up more towards being able to spam abilities that you can with Draining Whip.
Or simply nerf Draining Whip is an option too. It doesn't really feel like a choice between these two?