When I reviewed a bunch of SSGs looking for the one for me, the differentiators were:
* What language it's in (assuming you want to be able to hack around issues if you need to)
* What intermediary formats it uses (EJS/jade/stylus/LESS/SASS, etc. - assuming you don't want to publish a site that looks like the SSG's demo page)
* How opinionated it is about its output pipeline. Some SSGs have fully-formed plugin systems, so you can choose your favorite minifier, add a transpiler, etc., which is powerful but more work. Others just do what they know how to do and if you want to add steps you'll have to post-process the output.
That said, running through the "publish a hello-world page with default theme" was pretty much the same for every SSG I tried. You have to really start futzing with things to see the differences.