It's the quick and easy path to power. Many times it has been said that the Jedi's Path is a long and difficult one. Those who join the Dark Side want power and they want it right away. It doesn't matter which side is more powerful, it's about amassing as much power as you can as fast as you can.
See, I don't agree with that.
I think a lot of people who choose the dark side do it because it offers less restrictions. Less rules saying what you can and can't do. I don't think most do it just because it's the "easy" way to power. I think that's just a huge side benefit for many.