I asked myself the same question a few days ago. After extensive research:
"There is no better system for organizing CSS"
I have little experience with LESS, although SASS makes more sense to me.
You do not need Ruby on the server , since SASS simply installs it on your computer and downloads / synchronizes the output css files to your server as a regular css file. (SASS can support huge sites, the usual (reduced) static css file = less work on the server).
The SASS website mentions that the "SASS" syntax is shorter than the new SCSS syntax . Many will argue, but the indentation is beautiful, there are no brackets and semicolons, as well as import as + and mixins as =, etc. The SCSS syntax was simply created to compete with the LESS syntax. I tried both and for me the SASS syntax is much more fun, but this is just my POV.
The text editor and its settings that you use play a huge role when choosing between SASS and SCSS syntax . If you don’t know how to highlight 2 spaces using a shortcut, you can spend all the time twisting your face up in the SASS syntax code. SCSS will not give you errors for indenting from the place, you can be as dirty as you need (just like css css), up to a personal choice, although Chris Epstein made sense when he thought about personal and team work (in the comments):
http://thesassway.com/articles/sass-vs-scss-which-syntax-is-better
SCSS is populated quickly, as by default, so if you are going to learn the SASS syntax, you will probably find out in the end, as all new resources are directed to SCSS. (BTW you can convert SASS syntax to SCSS vice versa)
So, this is preference. Try them all, including LESS. (Compass is excellent)