libsass is - understandably - a few versions behind the Ruby implementation. Most of the project teams I've been on working with Sass have opted for the Ruby implementation as a consequence.
> In fact, Ruby Sass will not release any new features until LibSass catches up. Once it does, there will be feature parity between the two moving forward. Soon, we’ll have the blazing speed of LibSass with all the features of Ruby Sass!