[Raynor]

Originally Posted by Boromir88

I've often compared the Saruman-Sauron relationship like the Hitler-Stalin relationship.

I believe that it was more of a superior-subordonate relationship; or at least both feigned it was - because soon Saruman would begin to deceive Sauron's agents, but Sauron didn't have yet the strength to avenge that.

Originally Posted by Holbytlass

Saruman stepped onto Middle-Earth on the side of good, head of the white council, follower of Eru's plan. Somewhere along the way, he wanted to be mightiest, strongest, most powerful being, which doesn't always equate to evil.

I agree; in fact, that is true for both Sauron and Saruman:

Originally Posted by Letter #181

[The istari] were also, for the same reason, thus involved in the peril of the incarnate: the possibility of 'fall', of sin, if you will. The chief form this would take with them would be impatience, leading to the desire to force others to their own good ends, and so inevitably at last to mere desire to make their own wills effective by any means. To this evil Saruman succumbed. Gandalf did not.

Originally Posted by Letter #183

[Sauron] had gone the way of all tyrants: beginning well, at least on the level that while desiring to order all things according to his own wisdom he still at first considered the (economic) well-being of other inhabitants of the Earth.

Originally Posted by Notes on motives in the Silmarillion, i, Myths Transformed, HoME X

[Sauron] still had the relics of positive purposes, that descended from the good of the nature in which he began: it had been his virtue (and therefore also the cause of his fall, and of his relapse) that he loved order and co-ordination, and disliked all confusion and wasteful friction.(It was the apparent will and power of Melkor to effect his designs quickly and masterfully that had first attracted Sauron to him.) Sauron had, in fact, been very like Saruman, and so still understood him quickly and could guess what he would be likely to think and do, even without the aid of palantiri or of spies; whereas Gandalf eluded and puzzled him.

Originally Posted by Mansun

Would Saruman then, after finding out that if he could not master the Ring, hand it back to Sauron if he ever got hold of it?

I doubt that:

Originally Posted by Council of Elrond, FotR

Why not? The Ruling Ring? If we could command that, then the Power would pass to us. That is in truth why I brought you here.

Originally Posted by Kuruharan

Of 'mortals' no one, not even Aragorn...Of the others only Gandalf might be expected to master him-being an emissary of the Powers and a creature of the same order, an immortal spirit taking a visible physical form.
We have to assume that Saruman fell into the category of "Of the others" even though he too was an emissary of the Powers.

I think a great deal of debate has gone over a false dilemma. The initial quote looks like this:

Originally Posted by Letter #246

In his actual presence none but very few of equal stature could have hoped to withhold it from him. Of 'mortals' no one, not even Aragorn.

The passage in question doesn't concern who would master the ring _in general_, but who could master the ring in Sauron's presence. Maybe only Gandalf can master the ring in such a situation, but that doesn't exclude the fact that others can master the ring properly, should Sauron not be in the vicinity.

Originally Posted by Smaug

Would Saruman with his own armies from his base in Isenguard have battled Sauron for power in ME?

At the council of Elrond, as Boromir88 I believe quoted, Saruman is stated to have pursued a veiled policy of overcoming Sauron:

We may join with that Power. It would be wise, Gandalf. There is hope that way. Its victory is at hand; and there will be rich reward for those that aided it. As the Power grows, its proved friends will also grow; and the Wise, such as you and I, may with patience come at last to direct its courses, to control it. We can bide our time, we can keep our thoughts in our hearts

In the introduction to LotR, Tolkien gives an alternate view on how Saruman would challenge the leadership over M-E, should Sauron be enslaved:

The real war does not resemble the legendary war in its process or its conclusion. If it had inspired or directed the development of the legend, then certainly the Ring would have been seized and used against Sauron; he would not have been annihilated but enslaved, and Barad-dur would not have been destroyed but occupied. Saruman, failing to get possession of the Ring, would in the confusion and treacheries of the time have found in Mordor the missing links in his own researches into Ring-lore, and before long he would have made a Great Ring of his own with which to challenge the self-styled Ruler of Middle-earth.