I haven't installed Quick Silver in many years, so my understanding may be dated, but at the time it didn't offer many intuitive actions, such as copying the calculator result to your clipboard.
Alfred doesn't treat plugins as first class citizens, only alfred can have the top level search space, all plugins have prefixes. Plugins in Zazu are first class citizens, there is no built in behavior so you have more flexibility with what you want.
Zazu is also cross platform, so switching between Mac and Ubuntu for instance wouldn't hurt your workflow. Alfred and QuickSilver are Mac specific.
Alfred doesn't treat plugins as first class citizens, only alfred can have the top level search space, all plugins have prefixes. Plugins in Zazu are first class citizens, there is no built in behavior so you have more flexibility with what you want.
Zazu is also cross platform, so switching between Mac and Ubuntu for instance wouldn't hurt your workflow. Alfred and QuickSilver are Mac specific.