Apart from what has already been said (politics, basic UI) there are a couple more things worth mentioning:
Kbin’s interface is muuuch more customizable than lemmy’s: browsing form a web browser (desktop or mobile) let’s you modify your viewing experience as much as any mobile app for lemmy (but lemmly itself doesn’t). From infinite scrolling vs pages to font sizes and such.
kbin allows for (mastodon-like) boosting of posts, which is like a super-upvote that lemmy just doesn’t have.
on kbin you can subscribe to mastodon users aka federate with mastodon. Something that lemmy also can’t.
Other than that only personal taste matters in the end, and both federate with eachother, so enjoy it from wherever you are.