I feel that the ‘Latest posts’ homepage is perfect for conversations just as Discourse itself encourages.
But for the first time visitor (not-logged-in users), a ‘Categories & Latest posts’ gives a better idea of what the forum is about, as the ‘Categories’ section can define really well what you can expect there, while also having a peek at the ‘Latest posts’ on the forum.
This could also have some SEO implications as the search engines will crawl the ‘Categories & Latest posts’ homepage, which you can control a bit better in terms of keywords, text on page through the ‘Categories’ section. Also, the categories/sub-categories are linked directly from the homepage (better link flowing).
This could be the best of both worlds.