Hey, I am the Sword author. Harp development and Sword development have begun almost simultaneously. You should stick with Harp if you use typical NodeJS tools for templating (like Handlebars and Stylus), and use Sword if you prefer Ruby ones.
Sword indeed looks very cool, and yes, it is almost identical to Harp.
People should really pay attention to this approach because I'm fairly confident this is where static web servers/generator are headed. By lazily compiling only the assets that are needed when requested we save a stupendous amount of cpu cycles and we can keep the development workflow much more natural and rapid. Regenerating the world as Jekyll does, is ridiculously slow, awkward (spews out static files everywhere), and still requires a static web server to serve the files.
Today many people are manually setting up pre-compiling with tools like Grunt and Make to do their pre-compiling but I don't expect this to last much longer due to the cognitive overhead of maintaining these scripts and the lack of features such as layouts and partials. Not sure if sword supports this but I couldn't imagine harp without it.
@svoroval - Best of luck with Sword. Looks like we are on the same page :)
@taylorlapeyre - FWIW SASS support in Harp is right around the corner. We held off until node-sass didn't require a compile step to be installed. It now meets that requirement.
I may actually prefer this better because of the scss/sass support, though.