Ems is the fusion of two ms... And according to itachi, It grants the user "A new unique occular jutsu". With the ems the user doesn't have to worry about losing his light.
Sasuke and Itachi , both has same techniques....so it doesnt matter or lets it shudnt bother us ..thinking abt the new occular tech that user gets after geting the EMS
According to Itachi, apparently you unlock a new ocular power. I'm pretty sure he told Sasuke this. (correct me if I'm wrong) We didn't get to see anything new from Madaras EMS because he was forced to switch to the Rinnegan to absorb Narutos Rasenshuriken. But I think that was Kishis way of saving Sasukes power as a surprise. We'll just have to wait and see.
perhaps everyone saw that last power up sub-consciously... Okay, here's my best guess... I bet that ems last power up is....... The rinnegan! Think about it for a second, tobi said sasuke could surpass nagato and i don't see that happening without the rinnegan moreover he said something about synchronising sasuke with gedo mazo of the outer statue which is not possible without the rinnegan not only that, even kabuto says he wants sasuke... I beleive the rinnegan is the reason why they both want sasuke.