Saruman had already been corrupted, first by his own pride and then by Sauron through the palantíri. The One Ring would have made him even worse. Even if Saruman could wrest control of the Ring from Sauron (unlikely, as Sauron had already shown his will to be the stronger) the wizard would have become a new Dark Lord.
The funny thing is that he would have been even worse than Sauron. Sauron's Arda would have been little worse than North Korea, while enraged Saruman would have turned it into a wasteland.