I'm also a bit confused. Personally, I'm using Lemmy because it's been around for longer. Kbin seems really nice in the UI/UX department, but to be honest I'm not a fan of its tech stack.
This is the beauty of the fediverse. With open protocols I believe that there's going to be interoperability between whichever platform you choose.
I really hope your project or some of the other alternative frontend projects take off for Lemmy's sake. I really feel like lemmy is being held back by its UX right now.