The Power of Isengard

[The Saucepan Man]

That letter suggests to me that, in circumstances where Saruman held the Ring, Sauron would probably prevail over him in a straight fight between the two. I had it in mind when I said:

It is conceivable that Saruman would have a chance, but the smart money would probably be on the Red Eye rather than the White Hand.

Saruman's only chance would be to muster a sufficiently large force to hold off Sauron's armies. That would require an alliance with the Free Peoples but, by that stage, Gandalf had already spread word of his treachery. As I said, they would surely only have feigned alliance with him to gain a chance of seizing the Ring.