They are both awesome and compare quite well :D I think its mostly a question of the syntax you prefer (and possibly the way you'd like to manage the dependency, with ruby gems, npm, or manually).
IMHO Stylus is an overall win over SASS. The ability to write transparent mixins for my team (made by people not wanting to know how to implement `display:inline-block` on IE) is so great I can't be happier.
I've been able to completely replace SASS on Liferay [1] themes development and raised my team's productivity a lot.
http://learnboost.github.com/stylus/docs/bifs.html
Stylus is amazing