The good news is that by the nature of federation, the only factor that's really going to matter is which software you're more comfortable with. If you decide on kbin, you can still federate with all of the Lemmy instances out there and vice versa. And it's not limited to just those two services either. You can follow Mastodon users or any other ActivityPub-enabled service too.
So even if there's a clear winner between Lemmy or kbin public instances, as long as the projects are getting updated and one gives you an experience that gels with how you like to use it then go wild.