Shadow Blades provides very little burst, unless you're running Subtlety and you've got the Find Weakness debuff running. Killing Spree is an abominable source for burst, due to its completely random nature. You're lucky if you actually manage to hit your target, never mind the problem of actually surviving the Killing Spree yourself. Chances are, the only one who's going to die will be yourself....
Killing Spree is a horrible cooldown due to the fact that you lose control over your character for too long a period of time without compensation. You're not granted immunity to any source of damage, nor can you get rid of your excess energy. It fucks your character and it fucks your rotation. It's simply not very well designed.
Shadow Blades is bad and should feel bad for the reasons I've mentioned in my previous post.