I've not seen any real comparison between LessCSS and SCSS (i.e. the new Sass syntax that became available with Sass version 3.0) There were some real differences before that, but that's not nearly so clear to me anymore.
At present, we've decided that there's no reason to use LessCSS as we already use Haml and there's nothing that Less offers that gives us a reason to switch from recent versions of Sass. I'm not even sure what the differences are now.
Any bloat caused by the more verbose selectors seems to be easily offset by the lack of repetition, and the maintenance is orders of magnitude easier to manage. At the end of the day, you're still talking about maybe a couple of extra kb, which ends up being a negligible number of bytes post-gzip. In exchange, you get massively easier-to-manage stylesheets and a whole lot less developer time spent to make things happen.
Over the past couple of weeks, I've been refactoring a very large stylesheet with SASS.
Original size: 138.8k (gz: 23.5k, 16.9% original size)
Sass-generated size: 105.9k (gz: 18.2k, 17.1% original size)
Very, very easily worth the transition, and as a bonus, the stylesheet is factored into a bunch of partials, each relevant to different parts of the site, so when I want to tweak something in comments, I just open _comments.sass and hack there, rather than trying to find the right section in a monolithic stylesheet.If you want to just dip your toes in, look at SCSS. It's a superset of CSS, so you can take your existing stylesheets and start using more advanced features.
That said, I prefer pure SASS. It's cleaner, IMO, forces better organization, and is less typing.
It seems like a potential issue with less/sass could be producing gigantic css files.