Ask HN: Should I blog on Medium for my open-source project, or self-host?

both.

Use both medium absorbs a lot of SEO that Google would otherwise send straight to you

I think it vastly depends on your users. Generally, I would recommend self-hosting on the same domain as the project landing page or similar. It's unlikely in my opinion that users would bother to read your posts on medium.

I've found Medium incredibly useful for my open source blogging. First and foremost, it saves me from a ton of hassle picking a platform and theme for something like Wordpress, since it looks great on pretty much every device out of the box. It also has some great features that let people from the community submit posts to our publication.

Also, Medium is a great way for people to follow a variety of blogs - so not only do we still get all of the reads we get from normal sources like Twitter, sharing, etc, but you get a lot of people reading your posts from their Medium feed. In fact, for a lot of posts most of the views come from our Medium subscribers.

I agree with other commenters that setting up a custom domain is a good idea if you think you might want to move to some other platform in the future, especially if Medium is not around forever.

Agree. Specifically as the original question concerns an open source project I think GitHub pages makes perfect sense. Otherwise you can likewise deploy Jekyll-generated site on own host (or move from GitHub pages later).