They're neither different nor similar. In fact they work together, with Alpine managing client side reactive state (NOT app state, just interaction) and htmx managing the actual request model. That's why the htmx docs often refer to Alpine. They should be used in combination, not to displace each other.
This is also comparable to how Stimulus and Turbo work together, although I found to like using Alpine in place of Stimulus with Turbo and that combination works just fine as well.
The purpose of htmx is to enable the server to send fragments of markup to update the dom in response to UI events. Alpine.is is purely client side, you may not even have an application server.
