These are just two different software projects that a Threadiverse instance can use. They federate with one another, so it doesn’t matter all that much if you have an account on a Kbin instance, or a Lemmy instance. The differences are in the interface, some functionality, and the tech stack used (Lemmy is written in Rust; Kbin in PHP).
There are 100+ instances of Lemmy, and ~10 instances of Kbin. Kbin is a much younger project (hence it might get missed), and it’s main instance, kbin.social seems to be experiencing more issues with the wave of new registrations. If you want to try Kbin, fedia.io might be a good instance to check out.