First off I’ll assume Zoro is not blocking King with his swords because that makes a big difference. Sanji wasn’t blocking anything, he just ate it full force. When Zoro is blocking with his swords it puts the brunt of the attack on them and not his body. It’s like hitting a mail box with a baseball bat and the vibrations hurting your arms vs actually shattering your arm by punching that same mail box. Option A hurts yeah but it’s a hell of a lot better than option B.
That clarified it’s very reasonable to say right now that Sanji with RS has better pure “defense” than Zoro in a scenario like that. Zoro would absolutely tank King’s attack but he’d suffer more damage. But that’s what Zoro does. If we think about it in video game terms Zoro has an ungodly amount of HP. He can take incredible damage and keep trucking along. Sanji with his raid suit is like a character with really high armor but under that armor he has a lot less HP. Advanced armament haki would be much more crippling against Sanji than Zoro bc Zoro isn’t relying on a hard outer shell to tank damage. They are both very tanky characters right now but in different ways.
So I think King’s attack deals significantly less damage to Sanji but that’s not to say Zoro can’t tank more punishment over to course of an entire fight. Zoro will just be bleeding a lot more throughput said fight. We’ve seen that from Zoro time and time again in his fights and even the Kuma incident. You can basically cause him enough pain to kill him 3 times over and he keeps coming. This is not a who is better Zoro or Sanji thing for me but just a cool way to look at how two different characters handle the same problem differently based on their own unique tools.
Just thinking about the subject, there are 3 ways to be a tank. Sanji, Zoro, and Marco each do it a different way but are all incredibly hard to put down right now. Marco does it with his regen, Zoro with sheer willpower/toughness, and Sanji with high armor now that he has his raid suit.