This goes both ways. Zoro fanboys who used to downplay King now hypes him like crazy.
Now Luffy fanboys are cowering in fear in thought of King having one or two stats higher than Kaido.
First, Kaido's wind attack got deflected casually by Zoro while King's sent Zoro flying.
"But but nonsense King can't have stronger attack than Kaido cuz King admires his strength!"
Now King no sold Zoro's tatsumaki, protecting himself with the wing. While Kaido even in dragon form thus have his dragon scales, got wrecked, screaming, coughing blood, and white eyes after same Tatsumaki.
"Bbut but Kaido is most durable"
No. His durability crumbled after advanced CoA came to the story. Scrubbards managed to wound him, scrubbards managed to make him screaming.
So conclusion is
AP wise: Kaido >~ King >> Dragon Kaido.
Durability: King > Kaido in any form.
The stat Kaido has over King will be endurance. He endured fighting scabbards, 5 Worst Gens, Yamao, Momo and now Luffy again, while floating Onigashima.