I’ve used both, I recommend MkDocs over MdBook for most people. MdBook is simpler and easier to get started with, MkDocs has more features, more plugins, more themes. Both MkDocs and MdBook are easy to use. IMO, MdBook is a little too simple.

Recommend using the material design theme for MkDocs as a starting point. If you are working on a Rust project, use MdBook instead. If you have lots of docs / multiple projects / multiple versions, use Antora. If you want cooler landing pages, use Jekyll.