I use both as well. Karakeep is a fantastic bookmarking app and could definitely work as a “read it later” application, but I think the differentiator is how they approach content. Karakeep more or less preserves the link and all content, but Readeck provides the content in a simplified version without ads/margins/navigation menus/etc. this also allows you better customization of fonts/sizes/etc.

When I want to bookmark a website for access/interaction later, it goes to Karakeep. When I want to save content to read later it goes to Readeck