Are siths stronger than jedi?

It always seems that the Sith have the upper hand against the Jedi, even though since the 'Rule of Two', there are only two Sith and countless Jedi. So is the Dark Side of the Force stronger than the light?

Or is it due to the fact that the Dark Side is more chaotic and aggressive, so it's more useful in battle?

Also by stronger I mean, as in battles and overall use of the force during duels.