While Less and Sass are really similar products that do pretty much the same thing; I switched my site from initially using Sass (scss) to now using Less for three reasons:

a) Bootstrap uses less and I'm using bootstrap.

b) It was trivial to go from sass to less. Mostly just renaming files to have the right suffix.

c) I'm sure someone can argue differently, but I really think that the mixin syntax for Less is significantly better than Sass. I don't use mixins all the time, but this is really important if you want re-usable code. With Less, you don't have to declare @mixin and then @include it. Instead, any less block becomes a potential mixin.

That said, Sass has a compressor/minimizer and Less doesn't really have one. I had to integrate clean-css into my builds. It would be nice if Less had this by default.