HubL Template modules are locked and cannot be unlocked for editing in the page or blog editors, meaning that they compile when the template is published. These modules are ideal for adding editable fields/markup to a drag and drop template.
HTML modules compile when you publish the template and again when you publish content. These modules are ideal if your team needs to be able to edit the HTML/HubL from within the actual content (blog post or page).
While both modules support HubL, if you are adding modules using HubL to a drag and drop template, it is recommended to use HubL Template modules, custom modules, or a locked HTML module.