[{quoted}](name=Captain Rand0,realm=NA,application-id=3ErqAdtq,discussion-id=L0VGbqpw,comment-id=0000,timestamp=2016-02-19T01:08:34.450+0000)
Well the difference is that Zhonya's protects you for 2.5 seconds guaranteed, making or breaking a play very often. As a carry, Zhonya is almost perfect assurance vs instantly dying for 2.5 seconds. QSS is simply a CC break/Debuff removal. Now that's nice vs something like Zed's deathmark or Lissandra's Ult, but what happens when Yi Alpha Strikes you or Tryndamere spins to you. What can QSS do for you there? What will QSS really do vs Mundo munching your face off? Negate one slow, couple seconds later another? What will QSS do for you vs veigar's ult when it's about to hit you for 1400?Nothing.
Now I don't play ADC at all, however Zhonya's generally has more uses and more saving power than QSS. But, QSS can break CC, that which Zhonyas cannot. So it has perks so it's not straight inferior, but it is undeniably lower on the power spectrum.
Zhonyas is a stronger item with regards to saving you from more situations. HOWEVER -- you have the complete item which costs soooo much gold before you get that insane active. QSS on the other hand, as a defensive item, becomes effective with a relatively low cost before you upgrade it to mecurial