Yoda says it best in The Empire Strikes Back.

YODA: A Jedi’s strength flows from the Force. But beware of the dark side. Anger… fear… aggression. The dark side of the Force are they. Easily they flow, quick to join you in a fight. If once you start down the dark path, forever will it dominate your destiny, consume you it will, as it did Obi-Wan’s apprentice.

LUKE: Vader. Is the dark side stronger?

YODA: No… no… no. Quicker, easier, more seductive.

The dark side is like a drug. It’s easy to use and gives you immediate gratification. But it’s not stronger. The light side is more difficult to master and requires more time and patience but the strongest light siders will almost always outclass and best the dark side. This is because the Force itself is inherently light. The dark side is a twisted and corrupted version of the Force, a cancer. The Force seeks balance. In Star Wars, balance is keeping the dark side at bay. That’s why the Force created Anakin and awakened in Rey. Anakin’s brought balance by bringing the dark side down and Rey kept it at bay.

So in the short run, dark siders do seem to be more powerful but that’s because they gain power fast, but a dark siders ceiling is much lower. Whereas light siders need more time to get more powerful but their ceiling is typically higher.