The problem is that Gandalf explicitly refuses the ring for fear he would find it useful and thus be corrupted by it.

Like Gandalf, then, except smart enough not to pass up such an awesome opportunity to do so much good :D.

Incidentally, there's an essay by Tolkien where he explores the differences between the motivations of Morgoth and Sauron: Notes on motives in the Silmarillion. Some excerpts:

Thus, as "Morgoth", when Melkor was confronted by the existence of other inhabitants of Arda, with other wills and intelligences, he was enraged by the mere fact of their existence, and his only notion of dealing with them was by physical force, or the fear of it. His sole ultimate object was their destruction. [...] This was sheer nihilism, and negation its one ultimate object [...] Melkor could do nothing with Arda, which was not from his own mind and was interwoven with the work and thoughts of others: even left alone he could only have gone raging on till all was levelled again into a formless chaos.

[...]

Sauron had never reached this stage of nihilistic madness. He did not object to the existence of the world, so long as he could do what he liked with it. He 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 coordination, and disliked all confusion and wasteful friction. [...] 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 the palantíri or of spies; whereas Gandalf eluded and puzzled him. [...] But like all minds of this cast, Sauron's love (originally) or (later) mere understanding of other individual intelligences was correspondingly weaker; and though the only real good in, or rational motive for, all this ordering and planning and organization was the good of all inhabitants of Arda (even admitting Sauron's right to be their supreme lord), his "plans", the idea coming from his own isolated mind, became the sole object of his will, and an end, the End, in itself.

[...]

Morgoth had no "plan"; unless destruction and reduction to nil of a world in which he had only a share can be called a "plan".