Saruman for or against Sauron?

[Kuruharan]

That "only Gandalf might be expected to master him" quote has been used a lot on the Downs to "prove" that only Gandalf could have mastered the Ring and thus Sauron. I will not do full battle with that argument, but will merely point out that Tolkien uses the curious phrase "might be expected," which implies that this is not a for-sure thing. I will also add that Saruman also fulfills the qualities of Gandalf that are mentioned in this passage: an emissary of the Powers, of the same kind as Sauron; and, as has already been mentioned in this thread, he was before Gandalf's fall and resurrection more powerful (or at least perceived to be more powerful) than the Grey Wizard.

So which is it? You are arguing both points.

The "might be expected" part implies that it was not a sure thing that even Gandalf would be able to defeat Sauron even if he mastered the Ring.

Let me do some added emphasis...

only Gandalf might be expected to master him.

This is a statement of the unique stature of Gandalf. I don't see how it could be taken any other way.

Please, feel free to explain why Saruman could have mastered the Ring and proved a match for Sauron. However, bear in mind that you have already mentioned part of the answer to this riddle...

(or at least perceived to be more powerful)

...and that Tolkien said elsewhere...

[Saruman], having lost any devotion to other persons or causes was open to the domination of a superior will, to its threats, and to its display of power. - The Palantiri footnote 14

For a variety of reasons, Saruman was not on Sauron's level and could not be expected to successfully contend with him.

Oh, by the way, there is also another thing relevant to this topic in this little piece...

Saruman fell under the domination of Sauron and desired his victory, or no longer opposed it.
-The Palantiri

I think this sheds some light on the original question.