Prime Nelson was truly elite. Not a smart fighter but extremely good at what he chose to do. Granite chin, would just keep coming at you no matter what, and always trying to land that telegraphed overhand right. And usually, eventually, land it. And everyone knew better than to go ground with him. If Nelson had gameplanned and grappled more, he possibly could've been the champ.
But now he is truly old and past his prime, weaker, gasses, can't keep the same pace... Mirko on otherhand has had quite a few vitamins and picograms to keep him athletic enough.