Interesting comments on the Extend page [1] on why LESS vs SASS:

Why LESS?

One of Bootstrap's creators wrote a quick blog post [2] about this, summarized here:

- Bootstrap compiles faster ~6x faster with Less compared to Sass

- Less is written in JavaScript, making it easier to us to dive in and patch compared to Ruby with Sass.

- Less is more; we want to feel like we're writing CSS and making Bootstrap approachable to all.

[1]: http://twitter.github.com/bootstrap/extend.html

[2]: http://www.wordsbyf.at/2012/03/08/why-less/