The language has more powerful functions available, or at least it did when I first compared them.

I also find Compass, Bourbon and Susy to be great assets.

The Chrome dev tools support is outstanding.

The biggest drawback is that there's no client side evaluation like less.js. This occasionally makes working with other people a bit tricky. I usually make a sandbox.css for them to fiddle with, and then incorporate their changes every so often.

Sass supports nested media queries and custom functions. It has extensions like compass, which creates cross-browser supported css with ease. I was used to do everything with LESS two years ago, but recently I needed to switch for SASS, because of that.

I use it primarily because I like the tab-indented syntax. I seem to be in the minority with that, but I've never liked CSS syntax.

SASS can also take a bunch of PNGs and auto generate a sprite map and it's CSS.

Less is not always better ;-)

It's the default in Rails, which gives it a large built-in userbase.