[A_Brandybuck]

I think, we cannot judge, whether a Nazgul is more powerful as an elf, because their strengths are lying in total other fields.
As Neithan pointed out, the Nazgul's dreadful weapon was the fear. They aren't really good warriors. Aragorn (as it was altready posted serveral times) banished five of them at Weathertop. He didn't kill them, because they are really difficult to kill (at least the body). But not very difficult to banish.
The Nazgul fear themselves a lot. They fear flowing water, fire, "Elbereth", their master ;-) ...
Banishing them is not the problem, if the person, who tries, have enough bravery to fight against them. There is the crux.
It is necessary for fighting against them, that the person have got a strong spirit, strong will power, like Aragorn have it obviously. He can fight against them and banish them. Obviously Glorfindel have these attributes, too.
Elves seems to have an advantage over Men anyway. Men are mostly more weak. A good example, where we can see the weakness is the Ring.
So, when we are talking about who is more powerful: Elf or Nazgul, then I would say, that a Nazgul is a more powerful weapon against not strongminded enemies (i.e. Battle at the Pelennor), but Elves are generally better warriors and could defeat (or banish) the Nazgul, when they are strongminded enough.