At this point there isn't really a difference between the two. They're both fairly close in terms of every-day features. I've used them both, I'd probably recommend Sass, because it seems to get more love (the Ruby community is bigger than the Node/JS community currently). The only thing propping LESS up still is Bootstrap, which is written in LESS, although there is a port of it to Sass.

It doesn't really matter which one you learn, the main thing is to pick one and learn it, because it will make your life so much easier. Once you learn one, it's easy to learn the other.