SASS vs. LESS
LESS became popular thanks to Twitter Bootrstrap and to the fact that you don't need Ruby to use it, which is a big plus for Windows users.I use them both on daily basis, SCSS & Compass for a Sencha Touch project and LESS & Bootstrap for web dev work, along side with CodeKit to compile them and the major difference that I see is the speed of compiling. The projects I work on are both pretty large and with many .scss/.less files but LESS compiles way faster for me and this is a big plus.Also, LESS is pretty useful for rapid prototyping since you can use the less.js and skip the compiling part altogether.