SASS is an extension to CSS to allow for easier readability and maintenance. It comes with two main syntaxes:
You can see a comparison between the two syntaxes in the code samples which have a "Sass" and "SCSS" tab here: http://sass-lang.com/guide
So to answer your question: In the Lithium scenario SCSS and SASS are actually the same 🙂 The former being the syntax to achieve the latter.
Here's some further reading on history giving a good comparison between the two.