no, the grade is irrelevant in this case, any sword can become a black blade
meaning any blade can become part of the most elite blades in existence.
At that point just how important is the make of the sword? especially if we're comparing great grade to supreme grade, you'd think the difference then is negligible, at-least that's what I think, you shouldn't' somehow be stunted because you happened to find a lesser grade, despite making a black blade with your own strength, they are literally on another league entirely.
You're assuming somehow that means mihawk was stronger simply because the make of his sword is of better quality. That doesn't equate to how strong their spirit/soul/haki is.
WB for example referred to luffy once as a mass of spirit but all talk. This ties into how we see scopper telling off zoro for not even utilising his haki, that's just leaking to waste. Mihawk and Ryuuma have mastered using their spirit in their swordsmanship, or rather their souls have become aligned with their blades to a level no one else has achieved.
As to who's spirit is stronger, I don't know.