I daresay my argument above is still valid. Of course Gandalf does not make a second attempt to challenge the Balrog -- he defeated him anyway. Pandora's very argument shows the flaw in her argumentation, or at least a blatant lack of differentiation: only because of their restrictions were the istari less in power. The fight between Gandalf and Durin's Bane is only a valid measurement if you bear that in mind. Conslusively, the stronger spirit arguably resided in Gandalf. See
here for more about that.