Shusui- one of the 21 grade swords, made unique by being a Black Blade
Enma- one of the 21 grade swords, made unique by having a forced haki drain
Neither sword is a step up from each other. Enma will be when Zoro pushes it up a grade, but until then, nope. They are both 21 Grade Swords with their own unique selling points. They're both better than Wado, which is just a well made sword without any unique features, but measured up against each other neither is a step up. Shusui is just easier to use because you don't need to be a skilled ryou user to fight with it.
If Enma was one of the 12 it would be a powerup. If Zoro had picked it up and said "with this sword I am stronger than I was with Shusui" it would be a power up. If Enma, say, turned its user invisible and gave them iron skin it would be a powerup. Instead it's the haki process Zoro is going through that's making him stronger. Enma's a training tool for that.
