Enma has no real powers of it's own it's completely depenedent on haki of it's user that is not what you call special at all. Loda used it to add more difficulty to Zoro fight it was huge nerf up until last moment Zoro decided to give it all of his haki. Shusui wouldn't give him all that trouble and Zoro would still grow at the same rate since the challenges he would be facing would be the same. He didn't need any special sword to pick up breath of all things to cut steel in alabasta the same would happen against King invincible durability
I'd say Enma is less efficient as it pretty much demanded more Haki from Zoro to do the same job Shusui could. But that meant Zoro's Haki grew.
Once Zoro gave it the Haki it needs, it did the job.
Both are special in the sense that with proper amount of Haki, you can do wonders with these swords. A non-meito probably wouldn’t achieve same result.
Since I am looking at CPUs recently, I guess I can use the analogy of current gen AMD and Intel CPUs. Intel sucks up significantly more power to give a bit of better performance.
The problem is Mihawk said any sword can be made black blade, which in my opinion is the peak quality of swords. I don't know if that implies grading of swords was a completely pointless task to begin with.