Great question. As I'm new to these things too, I forwarded it to ChatGPT 3.5. In summary:

Lemmy is designed for creating and participating in online communities, while Kbin is designed for organizing and sharing knowledge.

Lemmy is more focused on discussions and social interaction, while Kbin is geared towards structured content creation and collaboration.

They seem to have more in common than they differ, especially for users who only read posts, visit links, write comments.

I also heard they are meant to be able to access each other's content, although that's currently not working.

Please correct what is wrong, happy to learn.

I prefer lemmy but you can use kbin if you prefer it. Its the same information at the end of the day since they can pull the same stuff