Tolkien leaves it deliberately unclear as to who would win between Witchy and Whitey. As has already been quoted here, Gandalf is very much unsure if he can beat him, though nor is he in despair.
Also remember that Tolkien never engaged in trying to rate how pwerful different things were against each other. it;s not like a comic book and it often does not follow any direct logic, with the myth and spirit of the piece being more important than anything literal. Odd things like the fight against the Balrog compared to the Spiders in the Hobbit illustrate this, excuses about not being able to use all their power are a. pretty weak and b. irrelevant to the argument anyway, because you can't start arguing about powers they cannot use, the argument revolves around what they can. I never bought that argument anyway, drama destroying as it is.
As part of this fuzzienss, the Witch-King that takes the field at Pelenor seems to be in a kind of battle mode, like the Witch-King of old in Angmar, as opposed to the fast-moving shade on Weathertop. There is no specified difference but definitely an implied one- possibly relating to purpose, distance from Mordor, the growing power of Sauron, taking new purpose after dissolution at the Ford etc.
This kind of thing makes these comparisons of various different events almost impossible.