We've used both Stylus and SASS. I've even contributed a few patches to Stylus. Ultimately, we decided that Stylus is a bit of a failed experiment in syntax/punctuation minimalism; SASS is much nicer to work with.
The key problem is that Stylus doesn't make a clear visible distinction between control structures, mixins, and other meta-level constructs. With Stylus, you can trivially override built-ins and it's very difficult to "go to definition", so to speak. SASS has very obvious symbols which call out the presence of preprocessor stuff. This makes it much easier to visually scan stylesheets and map what you're seeing in the inspector to what you're seeing in the source files.
Simply put, because the gap between CSS and the preprocessor language is so narrow, it makes sense to call out differences. Unlike, say, C and x86, it's too easy to look at Stylus and CSS side by side and be confused by their similarities.
Do you know if the Stylus compiler works any faster than the SASS one? Long compile times were an issue for me when I was using SASS a year or so ago. Although come to think of it, maybe I could've hacked a workaround by splitting my CSS into more than one SASS project.
During an experiment, I hit the 4095 selector limit in IE <= 9. Even with that combinatoric explosion of preprocessing hackery, compilation still only took about 2 seconds with SASS. Sooo, I'm not really sure what problem you're having :-)
Stylus is a nice concept, but when I found some issues or limitations, which is not unreasonable for a young project, it was not always obvious how it should actually work.
SASS is a more mature project with a solid design.
The key problem is that Stylus doesn't make a clear visible distinction between control structures, mixins, and other meta-level constructs. With Stylus, you can trivially override built-ins and it's very difficult to "go to definition", so to speak. SASS has very obvious symbols which call out the presence of preprocessor stuff. This makes it much easier to visually scan stylesheets and map what you're seeing in the inspector to what you're seeing in the source files.
Simply put, because the gap between CSS and the preprocessor language is so narrow, it makes sense to call out differences. Unlike, say, C and x86, it's too easy to look at Stylus and CSS side by side and be confused by their similarities.