They are equally powerful from star wars esoteric pov.

However due to the nature of the dark side and also the training regimes of dark siders its easier to find a powerful dark sider,than a powerful lightsiders.
Also dark side powers are more focused on expressing that power visibly so a very powerful sith sorcerer can overcome and defeat equal jedi master,just because jedi powers are focused in different direction.

Weak sith and darksiders in general can't survive the training to become full-fledged sith and die.So you don't see mediocre sith.

The most contributing factor is the force affinity of the given individual and also his conviction in the part of the force he uses.Not the dark side or the light side per se.

Also would like to point out that an exemplary Sith Lord have somewhat better knowledge of the force because they are not afraid of the light side ,they just choose not to use it or something like that.While jedi are too dogmatic to delve into all force mysteries because of fear of dark side corruption.

Conclusion:
It can be said that Sith delve deeper into force mysteries in general and get corrupted by it,while jedi stay save with clear conscience but restrict themselves to what they can achieve with the force.A conclusion that can be drawn is that any force user can be jedi,while only the strongest can be sith.This of course doens't mean that there aren't awesome powerful jedi,it just means that a big number of the jedi order would be too weak to become sith or survive a sith training.Also if the best of the jedi order somewhat became sith and started using all the force including the dark side to its full potency,they would become more powerful than they were while being jedi.But in the usual case that won't happen cus as said above strong conviction is needed.