I have been using LESS for a while now and recently I had to amend my friends code which was written in plain old fashioned CSS, and I realized just how much faster having a CSS preprocessor as part of my workflow has made me. SO MUCH FASTER.
Both allow you to create variables, nested variables, Mixins, Selector Inheritance and Functions & Operations with the main difference (that I can see) being that SCSS needs Ruby installed where as LESS does not.
I also use Crunch as an editor and to compile minified css. Oh and on a complete side note if you ever need to easier understand or work with messy, illegible css check out Processor, it will have your code looking amazingly pretty in no time.
Both have solid communities behind them so which ever you chose is up to you, but if you have not tried working with one before I would highly, highly recommend it!