The manga tells us King's durability and defense is beyond Kaido.
King clashed direct with Zoro's ultra tiger, stronger attack than dragon twister, with his body and sustained no damage to his wing or head.
King took a Death Liong Song to face, Zoro's go to attack when dealing with characters or objects hard to damage or cut, and King sustained no damage.
Manga facts supports King has a greater durability and defense than Kaido.