htmx is a relatively low level extension to HTML, by design. Unpoly offers a lot more structure, such as layers, which you would need to implement on your own or using another library (e.g. tailwinds or bootstrap) if you are using htmx.

This is reflected in their overall sizes: 11k for htmx, 41k for unpoly.

I don't view either as better in general: they are two different approaches to building hypermedia-oriented applications, one more structured and one less. Which one is better for your particular application depends on your own engineering needs and tastes.