Implementation aside, what's your preference? LESS or Sass?

I’m looking at deploying a solution that would just make LESS or Sass (perhaps with Compass) available from the server (LESS apparently can run server-side). Do you actually use one, either, or both, and why?

Personally, I’m mostly interested in CSS variables and nested rules, but mixins and some of the function-like behavior seem pretty compelling as well.

The implementation would be server-side and as transparent as possible. For example, suppose you had this in the HTML:

<link rel="stylesheet" type="text/css" href="style.less" />

It would automatically compile the CSS as appropriate (e.g, if it has been changed), create a “style.css” file and then rewrite the HTML to your typical:

<link rel="stylesheet" type="text/css" href="style.css" />

The idea would be to make it just “work” and not have to worry about compiling either server- or client-side. I’m leaning toward prototyping it with LESS since it seems more broadly accepted, but is there a compelling reason to do Sass or Sass + Compass instead?