I’m on Lemmy because I’ve found a really nice programming-focused instance I like. But I was seriously considering kbin because its UI seems to be more polished.

But the great thing about the fediverse is that unless you are on a radical instance most others block, it doesn’t matter where you register, and even the choice between Kbin and Lemmy becomes an aesthetic/usability one. You don’t lose out on anything, you can subscribe to communities anywhere, post, comment, message people etc.