I started with Less as it was "less" painful to get it up running, and configuring tasks with Gulp/Grunt was easy and fun. Sass wasn't, at the time.
But as time followed on, I noticed that all the projects I followed switched to sass at some point, so I decided to give it a shot. Being on Windows mainly, I thought it was a pain to install Sass at first (either Ruby + Compass, or Grunt/Gulp with buggy plugins, or Gulp/Grunt + Compass...) and spent lots of hours trying to make the whole stuff work. Eventually I got myself PhpStorm and after a few minutes configuration, Sass was up and running :)
So basically now it's Sass, mainly for it's popularity, but also for its wider possibilities.