Seriously sasuke got EMS and hes useless agaisnt kabuto, Itachi got beaten by sasuke and only got MS.
Its amazing how kishi makes itachi so mutch stronger then sasuke just because of all the fanboys
Its because of experience and also that Itachi has always been better then Sasuke at everything since the start.
Just think, how long has Itachi had MS and how long has Sasuke have his MS and EMS? Itachi was also in Akatsuki for a long time while Sasuke was sitting around with Orochimaru wasting his time training to be Dr Snakes instead of getting better with sharingan and other more useful techniques. Itachi has proven to be smarter too in battles, always a step or more ahead and planning for events that will happen in the distant future. Now Itachi is also an edo so he is at his best, stronger then he was when fighting with Sasuke because he isn't half blind now and can keep his susanoo up for ages. + Itachi is not afraid of using forbidden techniques because he is already dead, also while we are on that point, Sasuke never even heard of Izanagi or Izanami until he saw them for the first time, didn't even know what susanoo was until Itachi showed him. He lacks a lot of knowledge about the Uchicha, which Itachi had plenty of time to research about MS and EMS.