Their actual fighting power appears to be somewhat similar since they fight for some time, and both end up perishing (at least Gandalf's mortal body dies). I would probably say that the balrog may have actually been stronger given the Istari factor of Gandalf... Gandalf won, but he had a sword to help him. Unarmed, he would have had a tougher time. Who knows how powerful Gandalf was in his natural from? I would guess a fair amount stronger than his 'white' form which was still stronger than his 'grey' form.
One thing however, I think that Gandalf's real asset was his wisdom. As far as Maiar go, he probably did not stand out based on this strength (fighting ability)
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