Mastodon can interact with Lemmy as well as far as posting and commenting. On a basic level, kbin offers microblogging (your Mastodon-style posting) and a more sleek interface. The visual polish is a little better on kbin, in my opinion. Right now, I'd say the biggest thing is that kbin development is slower, so new feature rollouts are slower than Lemmy at the moment as Ernest works to make sure everything is stable as kbin grows. The API might be available now for third-party development of apps, but for a few weeks there, it wasn't, whereas that started almost immediately on Lemmy with API availability. But with Artemis for kbin in beta, it sounds like that might have changed. Otherwise, though different, Lemmy and kbin are both based on ActivityPub, hence why you can interact interchangeably with communities, users, voting, and so on. And maybe worth mentioning, kbin allowed community creation earlier than a lot of Lemmy instances, though that did change quickly.

I think it just depends on what you're looking for - a polished experience with interesting features (kbin) or more cutting-edge feature rollouts and updates more frequently (Lemmy). Of course, I don't know what the future holds for either platform, so that might change if Ernest gets more of a team on board.