I also think Kbin is way ahead of Lemmy in terms of integrating with the Fediverse at large. It's not perfect for sure, but in my opinion it builds on a better starting point. The real test of Mastodon integration for me will be when Mastodon finally introduces groups - it will be interesting to see how they integrate with communities in the Threadiverse.
I honestly also struggle to get an overview of what's happening in Lemmy development, to me Kbin seems more transparent. But both platforms are obviously fine, and Lemmy is still more mature for sure.