The two complement one another well: htmx for exchanging HTML with a server and alpine.js for pure front end stuff (e.g. toggling a class)

hyperscript, our other project, is a more direct competitor w/ alpine:

https://hyperscript.org

Alpine is a lot closer to jquery than htmx IMHO--I think of it in a very similar way as what people used to use jquery for all the time (adding event handlers and logic to DOM elements).