Less is the left behind brother of Sass.
Thats my verdict, you don't have to read on, but if you want to know why I think that, feel free to continue reading.
About 9 years ago, after a lot of playing around and trying to create my own preprocessor using php and generating css in php files in order to use "variables". Less came out! Oh yeah, what a dream. There were multiple ways of using it, as a preprocessor or just add the less.js to your project and let the client side process the css. For me this was easier and my experience with working in a proper way with a development environment and everything wasn't that far yet. At this moment I was still developing on a live server ("He, are you in this file? Can you close it, I need to fix something"). But LESS was amazing, I could finally use css in other ways and so much faster.