I am the author of compass so I definately understand Sass more intimately than Less. Please feel free to correct any misunderstandings I might have. Also note: I am comparing the versions of Sass and Less that they suggest on their main websites -- both have some very cool features and enhancements in the pipeline.
My initial preference was Less, because sass seemed framgmented with 2 syntaxes, and also less seemed more aesthetic, but I ended choosing Sass for the following reasons:
* Sass has Compass built over it http://compass-style.org/docs/ and this alone is the biggeast win over Less
Compass is a framework that comes with a foundation of components :
- compass core http://compass-style.org/docs/reference/compass/
- many plugins for different needs (search github for compass), special mentioning Susy liquid grid framework https://github.com/ericam/compass-susy-plugin
- with Compass I can monitor a whole directory of templates at once, and have them instantly compiled when I save from the editor, not sure if Less can do this
* Aptanta Studio 3 and few other IDE's know to syntax highlight sass and scss, not sure about less
* If you don't like the indentation syntax you can use the curly Scss, Scss and Sass are interoperable, you can call one from the other and vice versa
* The ruby version of Less is deprecated and I write a ruby based project (indeed, not all HN users are pythonists)
Moreover, I don't like the idea on compiling the templates client side (as in less.js) and waste precios miliseconds, nor use an external interpreter (node.js)
Disclaimer: I'm the author of Sass.
Since then SASS has grown the SCSS syntax, but haven't really given me a reason to bother switching.
Also: Less now has the even better less.js, which is handy for quickly throwing together less stylesheets and really fast on the command line compared to the old Ruby version (an order of magnitude faster for 4,000+ line files).
The most useful feature of LESS is simply nesting / indentation.
Since it's not syntactically different from CSS, you can start a LESS stylesheet from a standard CSS stylesheet and move forward from there with LESS.
Compass hasn't interested me much.