It's not just about having the power...

[Ibrîniðilpathânezel]

I think that there's another facet of the matter that isn't being considered: the fact that Saruman was actively afraid of Gandalf. In Unfinished Tales (The Hunt for the Ring), we are told:

Saruman soon became jealous of Gandalf, and this rivalry turned at last to a hatred, the deeper for being concealed, and the more bitter in that Saruman knew in his heart that the Grey Wanderer had the greater strength, and the greater influence upon the dwellers in Middle-earth, even though he hid his power and desired neither fear nor reverence. Saruman did not revere him, but he grew to fear him, being ever uncertain how much Gandalf perceived of his inner mind, troubled more by his silences than by his words. So it was that openly he treated Gandalf with less respect than did others of the Wise, and was ever ready to gainsay him or make little of his counsels; while secretly he noted and pondered all that he said, setting a watch, so far as he was able, upon all his movements.

That seems rather paranoid behavior for someone who feels secure in his greater power. It's fairly clear (at least to me) that in Tolkien's mind, Saruman was not secure in his own power, and was afraid that sooner or later, Gandalf would move against him. His fear of Gandalf is mentioned again, as is his eventual knowledge that Gandalf possessed one of the Elven Rings. I think that at the time he imprisoned Gandalf, Saruman didn't dare try to start anything he feared he could not finish victoriously. If he had felt confident of his ability to defeat Gandalf in a battle of power, I suspect he would have tried to at least wrest Narya from him (since by this point, he's certainly aware that Gandalf has it), to presumably weaken him and put Saruman in a better position to force information about the Ring from him. But he doesn't. He tries persuasion and intimidation, but both ring hollow, as does his attitude of superiority. Saruman talks the talk, but he doesn't walk the walk. He has armies and guards to defend him, but he doesn't do more than imprison Gandalf. Saruman's afraid of him, a fear compounded by paranoia. Such people usually project their own motives on those around them, especially those they view as rivals. Even in defeat, Saruman will not give up his staff or the keys to Orthanc, because he believes Gandalf wants them for himself. If such a person made no attempt to annihilate his enemy when he had him in his clutches, I believe it's because he felt he could not defeat him. Elsewhere in the same chapter, it's said that Saruman recognized the "great power and strange 'good fortune' that went with Gandalf." Perhaps Saruman felt that Gandalf had the assistance (or protection) of the Valar, and attempting to defeat or attack him through power might bring down their wrath upon him. Lacking the Ring, Saruman would have had no chance of avoiding that, and he may have viewed it as a risk not worth taking.