What is the difference between Less and Sass?

There is a Gist in which the differences and specificities are put in detail .

There is also this document which considers several more macro aspects between the two standards , which in the evaluation final SASS is higher, although the LESS has several things in its favor. I recommend reading the comments part too.

There is still a pattern called Stylus, very succinct and beautiful , but with little adherence until then, but that deserves attention.

In addition, there are at least 8 CSS preprocessors, known and semi-known, summarized here . Worth a quick read.

In any case, it's important to say that the choice should also be based on the amount of tools available to preprocess CSS on your favorite technology. For example, Bootstrap and web2py have their stylesheets made in LESS. Yeoman has more developed support for SASS, and so on.