When Stylus was created it was definitely more powerful than Sass. The syntactic flexibility of mixin invocation lets you define your own css properties and override how others work transparently. It also had a better story for defining in-language functions and whatnot.

Since TJ stopped working on Stylus the Sass guys have continued adding features. Using @extend can significantly reduce the size of the output CSS file, the ability to pass blocks into mixins was added, and SassScript roughly matches stylus' language. I consider Sass to be more powerful but someone using a different/older feature set could easily argue the other way.

The sass community is also larger/more prolific so you have sass frameworks (not that I generally use them but they're there) and things like the Team Sass github org doing interesting/random things.