[Iarephelion]

I suspect this is a question which cannot be answered with any real accuracy because of a contextual problem. Sauron and Gandalf, both of Maia origin, served very different purposes in Middle Earth. Sauron was loosed without any constraint upon the land and was received from Morgoth a legacy of might and destruction, a precedent if you will. Gandalf, as a servant of the Valar, was restricted in his actions in Middle Earth and thus appeared "less powerful." His task was original in its nature and he therefore had fewer physical resources at his disposal. Had the two been in opposition during an earlier age, I suspect the circumstances would be quite different. Given their respective situations in LOTR, Sauron would certainly appear to be the more powerful, but I believe this to be a circumstantial rather than an absolute representation of the relationship between the two.