They are similar. I think medium catered to bloggers, and gave a space to post easily, and get some extra viewership through their community. Substack caters

They are similar. I think medium catered to bloggers, and gave a space to post easily, and get some extra viewership through their community. Substack caters to more prominent writers/bloggers who want to charge for their content, and build a following, and handles the logistics of that. So maybe something like Patreon and Medium combined