I see both Jekyll and Hugo mentioned frequently in the "Using GitHub and X to host your website" space. Do you have any practical our technical reasons for preferring one over the other?
If you're not already set up with a Ruby development environment, I'd use pick Hugo over Jekyll. Jekyll is nice but getting all the Ruby junk going was annoying, IMO. Maybe it's trivial for someone better than me.
Having used both quite a lot, I much prefer Jekyll's template language as it's easier to use and more readable. It's a lot easier to install plugins to extend Jekyll as well. However, Hugo coming in a single binary makes it easy to get working when you come back to a site you've not updated in a while (my Jekyll toolchain was a combination of Ruby + Node + Gulp that felt brittle but could be improved with some work), it's suppose to be significantly faster (I've seen people saying it takes Hugo seconds to generate thousands of posts compared to minutes with Jekyll) and I've had less issues with Hugo's watch feature (Jekyll doesn't refresh when you change files sometimes for me). I don't use a lot of plugins or complex templates so the speed and simplicity of Hugo is a big factor for me.
Does Hugo support partials? An equivalent of frontmatter? Slim Lang?
It has partials, Front Matter as well and you can use Ace or Jade templates as an alternative but not Slim Lang as far as I know.
It honestly doesn't really matter. Both ingest Markdown, apply a theme, and spit out HTML which you then copy to your HTTP server. It's sort of like asking "which version control do you prefer? Mercurial or Git" or "which build tool do you prefer? clang or gcc." Your choice in the matter doesn't really impact the end result, just your own workflow. Jekyll is probably more popular simply because GitHub has it integrated with GitHub Pages.
I've dropped Jekyll just because of windows support. I've been using hexo and hugo in two projects and worked great!