I've used both, and been very pleased with both.
Mint stood out, last time I installed it, because every decision was easy and factual and about me (what time zone, what keyboard).
I essentially just pressed "next" a bunch of times.
Kubuntu was nearly that good last time I tried it, as well.
Between the two, I generally recommend Mint primarily because it keeps the messaging simple and consistent with the community.
Secondarily, because Mint doesn't have Snap (and I consider Snap bad, in a way that new Linux users are unlikely to appreciate until much later.)