I wasn’t talking about the grade in comparison to Shusui. The blade itself makes Zoro stronger simply because of the grade, and then whatever properties it has with it. It makes him stronger.
Shuisui and Ema are the same grade.
I see Ema only has a power up if you think that it is required to help Zoro to improve his armament hacky. By that I mean that Ema can easily access to Zoro hacky, In the picture you see that Zoro when call for the hacky get his arm drain but right after Zoro is able to balanced it properly.
I do think that Ema has other properties but for now is pure speculation
In the case of Shusui you right that was a power boost for Zoro and we have see what that sword was able to do(cutting like butter the dragon - cutting monet - cutting Hody etc)
Probably Shusui has other properties that we didnt see.
So so far I am not sure that Ema is in fact a better sword than Shuisui but it ll help Zoro to master the black blade :)