Let me say this first, because I don't think the OP knows this: Sith and Jedi are not equal to Light and Dark. Being influenced by the Dark side of the Force doesn't make you a Sith. Same is said for the light side of the force and Jedi, though it's more rare that this is the case. With the EU being considered: Sith teachings and dark-side power is far more potent than the restrained 'light-side' of the force, but it is far more unstable and always leads to corruption to the point where Sith always end up behind. The Jedi teach their padawans the path of the light-side, which is basically a fancy way of saying the reasonable side. Any time you stray outside of what the Jedi constitutes as the light side of the force, you are in jeopardy, and will inevitably, end up on the dark side, which is a path that will never end up good for you in the long run. There has yet to be a powerful Sith Lord who has achieved all that he/she wanted to achieve. Even Sidious almighty. That's not to say that the light side is weak. Anyone saying "Dark side is way stronger! Light side is for punks!" doesn't know the Star Wars EU. There have been countless times when Jedi Masters and Knights have proven to be way better than their Sith counterparts. To be honest, the only time the Sith are actually ever 'more powerful' than Jedi is when you're considering big name, legendary Sith.