I think you can achieve the same thing with both, but Unpoly is higher level, that means I have to write less attributes/code to achieve the same stuff I guess.
"Compilers" are also a nice way to attach custom code. All the helpers for form validations, the extremely easy way it provides to do modals, and layers in general. The navigation feedback (a'la turbolinks) when navigating across pages, the caching, the error handling. The up-keep for keeping a video player or anything else across pages transitions, and absolutely everything of this can be accessed from a JavaScript API in case you need to perform any of this from your own code and most of these APIs also trigger events.
Probably you can do all of this with HTMX too, but it is more work.