Apple and Oranges. They both were talented frontmen in their day.
But as for preferring them in Sabbath, I kinda have to go with Ozzy. While I agree Dio was a more technically polished singer than Ozzy (and his stuff with Sabbath was indeed very good), Ozzy's songs are the ones people think of when they hear the name Black Sabbath. Plus, Ozzy has a special one of a kind presence and mystique about him that sticks out.
When it comes to singing ability and vocal talents alone, Dio blows Ozzy out of the water. But as for frontman skills, showmanship, being a better fit for Black Sabbath, and who is more influential, that's a bit more difficult to determine. I would say both are equally important to the history of the metal genre.