Oh I think Nico's actually at peak power right now. Who is supposed to teach her magic? She's not gonna learn anything new and the Staff of One only gets less powerful unless you start studying other languages.
That said, the Staff of One is still more powerful than Wiccan.
Sister Grim wins.
I would say Wiccan. He is listed as one of the most powerful sorcerers in the Marvel U and he is still wet behind the ears. I am not too sure where Nico stands on power levels but she can only use spells once. So everything we have seen her do, she cannot do the same way again, which requires her to remember every single way she has cast a spell ever while at the same time learning new ways to do magic. Wiccan does not have that problem. So I give it to him.