I personally prefer Lemmy over Kbin for many reasons:
None of the Kbin names make sense. Why are communities called magazines? Threads are posts? It just doesn’t map to existing mental concepts very well.
Kbin makes you go through settings just to access your subscribed magazines.
Kbin exposes votes front and center in the UI. You can see who voted for what post / comment and personally I think voting should be private. Obviously, anyone can host their own instance and see the votes due to federation, but there’s a big difference between that and outright showing it in the interface.
Kbin’s UI looks a bit outdated and early 2000s to me. It looks like a mix of the old Facebook style and Digg.
Lemmy’s UI feels more performant, especially on mobile.
Microblogging feels like an afterthought. Makes the site feel like it’s shoving two different sites together.
Content wise, both Kbin and Lemmy federate with each other, so ideally you should be seeing the same amount of content, but there’s federation and load issues on both sides that’s preventing that from working correctly. I have faith it will be solved eventually as these are just early growing pains.
I will say that, personally, I’ve found more of the communities I want to join on Lemmy, but I am sure another person with different interests might say otherwise. Try both and make your own decision!