What's better - to post articles on your personal website or open writing platform like Medium?

Personal website for sure imo. For one thing, often your content will be behind a paywall on Medium so people won't be able to see it. For another, you won't get the SEO benefits. People talk about the distribution you get from Medium - I have never really found that to be super useful. Additionally, you can always cross-post to Medium (make sure to use the import feature) after you already published on your site. It's quick and easy to set up a Wordpress blog - you can do it in an hour or two.