Who defeated who: Stalemate. The published novels and website state that Sidious defeated Yoda, but this isn't the case. In fact, Yoda got the finishing blow. But Yoda was in a position where, to return against the Dark Lord, would be suicide. I believe, like Yoda with Dooku, Yoda would have won a prolonged battle.
Who was better: Depends on what you mean. Better at what?
Who was smarter: Sidious. He duped Yoda for about 14 years, and brought the end of the great Galactic Republic, and instituted the most powerful militaristic regime in galactic history. Oh, he definately won the intellectual war.
Who had the bigger impact: Yoda, of course. He is the more popular character by far. Probably second only to Vader. But as for the story, Sidious. Sidious converted the main character to evil, and caused the conflicts in which the story is centered around. The story needed a Sidious. The story did not necessarily need a Yoda (until OT times, where he was VERY important).