I have 3 AP left to spare in my build and im not sure whether to take 3 ranks of

No Mercy: You deal 10/20/30% additional damage to helpless opponents.
OR
Killer: When you kill a target, you gain a 5% Morale bonus to Doublestrike and Doubleshot for 15 seconds. Weak enemies will not always produce this effect. The Killer buff can stack up to 2/3/4 times. Stacks are reduced one at a time.

Any advice on which would of the two would be better and why? (I have a few options of consistently making enemies helpless, like blind, save for bosses, so it can be taken into account that No Mercy passive can be almost always applying)