Primary purpose:

Less – is a CSS pre-processor, meaning that it extends the CSS language, adding features that allow variables, mixins, functions and many other techniques that allow you to make CSS that is more maintainable, themable and extendable. (http://lesscss.org/)

Processing requirements :

Both – 3^rd party application or command line, Sass requires Ruby be installed, Less requires NodeJS

Key Features:

            Less is designed to be as close to CSS as possible

            Sass is designed to ‘make CSS fun again’ by taking the tediousness out of writing style sheets.

            Both have variables, nesting, mixins

Primary Strengths:

 LESS  

-       do the same things in fewer time

-       More simplicity and no useless features

-       LESS is very lightweight and easy to learn for third party developers who never worked with CSS pre-processors

-       There is a stable PHP-parser for LESS

-       No dependency to other languages like Ruby etc.

-       Works better with Bootstrap

-       Name space?

Sass –

            Selector inheritance

            exposes a long list of color functions not found in CSS

            supports unit-based arithmetic

            has conversion tables

            will let you define your own units (for futureproofing W3C changes)

            Sass output compression currently beats the google pagespeed css plugin output by a few percent.

            with Sass it's also really easy to extend on what others have built before you

 Issues, Drawbacks or Problems:

            Less does not provide any conditionals or looping structures. (It does provide mixin guards and pattern-matching which can be used to similar effect.

            Sass takes much longer to compile in non-Ruby projects.

            Less 1.7x I can do things Sass cannot. Like passing rulesets to mixins:

Additional notes:

With Sass: Need road-tested mixins with cross browser support and thorough documentation? gem install compass. Need really great responsive grids? gem install susy. Want to add some eyecandy and call-to-actions? gem install sassy-buttons.