lemmy is the more mature platform, whilst kbin is newer and more feature rich.

What it amounts to is that kbin can do things that lemmy can’t do, but the things that they both do, lemmy tends to do better. And as kbin is effectively in alpha at the moment, it doesn’t have much documentation, making installation and configuration a challenge.

The biggest point of difference in features is that kbin is aware of other fediverse content in a way lemmy isn’t. kbin and lemmy both talk to each other really well, but kbin also natively supports other types of fediverse groups (gup.pe, friendica and chirp). kbin also lets you see non threadiverse content, by attaching hashtags to groups. So if you set up say a cycling group on kbin, you can also make the group watch the #cycling tag, any any mastodon or other micoblogging content will appear on a special tab in your cycling group.