KD is a better player, but if I'm the Cavs I would take Mitchell over Durant all day every day.
He's far younger, pretty sure he's cheaper, his career is on an upwards trajectory, and he seems to be a quality locker room guy. He's somebody you can build your franchise around for the next decade.
Comparatively Durant is a better player, but he only has a couple of years left in him, he's a drama queen, he's likely to just demand a trade in 12 months if things don't go exactly how he wants.