At Sponge we use Less as the default, as we use Express.js as our webserver and it supports it straight out of the box via middleware. 

Its a super-set of CSS so if you just want to use plain ol' CSS syntax then that is just fine, but if you want to refactor your styles to use variables and mixins then that's okay too.

The only "problem" I sometimes encounter is when new devs discover nesting rules and go a bit crazy and build really deep, over-specific rules are that not reusable. 

I've never used SASS, but at a first glance the syntax is pretty much the same, just more $'s and less @'s.

I've just done a quick test of node-sass and it SASS works natively in Node, so no need for Ruby.