I've been trying both. I find Kbin has a better user interface, BUT it's slower. Possibly server specs, but probably partially because tech stack underneath - Lemmy is Rust, Kbin is PHP. Neither seem to make use of any search core like Elastisearch, which would be a big win for performance, but probably another administrative overhead/cost for the admins.
Kbin also doesn't have a mobile app yet, where Lemmy does, though I think Kbin's web interface on mobile is pretty good.