Interesting, I have been using mjolnir for some time for basic bindings (window resizing, jump to named windows). What does hammerspoon offer over it?
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.