Hammerspoon is actually just a fork of Mjolnir from a few of the more active community members.

It has a bit more modules and whatnot and emphasizes ease of use more than Mjolnir. It comes pre-packaged so that you don't need to bother with Luarocks and finding each and every package that you want. It is also under active delevopment. .Steven Degutis, the Mjolnir creator and maintainer, has said that he won't be doing anything other than merging necessary pull requests.

So really, for what you're using it for, probably not much. It's almost a matter of preference.