Chris Coyier has an awesome rundown of SASS vs LESS over at css-tricks.com. It's definitely worth the read.
As for some of your specific questions:
I work entirely with SASS/Compass, so I'm not intimately familiar with LESS's community, but nor have I really needed SASS's community. Their documentation is fantastic and has solved any problems I've run into thus far.
For what it's worth, though, here are the SASS vs LESS statistics that Chris has in his post, which I've updated for current numbers:
Number of open issues on LESS: 121
Number of open issues on Sass: 87
Pending pull requests on LESS: 13
Pending pull requests on Sass: 8
Number of commits in the last month in LESS: 49
Number of commits in the last month in Sass: 7
To note, these numbers were roughly flip-flopped as of Chris' writing in May, 2012. This says to me that they're both pretty equal with regards to development activity.
Technically speaking, Sass is older. It came out in 2007, while LESS came out in 2009. That said, the comparisons I've seen put both of them at pretty much the same level of "maturity" in regards to features and whatnot.
Both also have frameworks that bring more tools to them. LESS has Less Framework, and Centage (additionally, the Twitter Bootstrap is built with LESS). Sass has Compass, Gravity, and Susy. Both probably have more, if you dig for them, but those are some of the first that come up when you search.
So are there any real differences?
When it comes to writing it, not really. If you use the CSS-like SCSS syntax in Sass (instead of the more Python-like SASS syntax), you have only the typical minor syntax differences (
$), but for the most part, they're basically the same.
The two biggest differences in coding I found were a) how they handle units when doing math, and b) how they handled inheritance. When given something like
20px + 2em, LESS will drop the second unit and assume you mean the first (yeilding
22px), while Sass will throw an error (basically, type mismatch). With inheritance, LESS treats it like a mixin (I can't really explain it well, so see the inhertance section of this Tuts+ article for details).
Whether one is superior over the other kind of depends on how you'd prefer it to handle things.
The other biggest difference I know of is how and where each one compiles by default. Sass uses Ruby and compiles on the server, allowing you to store and send the compiled CSS file to the client. LESS, on the other hand, defaults to using the less.js script to compile the CSS on the fly. However, with the use of Node.js, LESS can compile on the server side in the same way that Sass does.
So, if they're basically the same, which one should you use? Well, unless you really love the Python-like SASS syntax, or really think that client-side compilation is the way to go, or you greatly prefer one's inheritance calling over the other, it's going to matter more whether you prefer to have (or already have) Ruby or Node.js installed.