In the stories of the Star Wars universe, the Sith always seem so much stronger (with leaders like Exar Kun, Revan, and Vader), but they can never remain that way. Great leaders do not pass on secrets or knowledge to those who will succeed them, so they can really only know as much as the Jedi currently do. They also have trouble remaining focused on cooperating against their enemies, so as soon as they gain a good footing they immediately start fighting amongst themselves. They may be the most powerful in the universe for a single generation, but they will then die out again until a good many generations later. <_<
So, I'll say the Jedi are better thanks to their consistency.