This argument has always been dumb.
Sanji trust in Zoro defeating King, so he has an idea of how strong Zoro will become during the fight, just like Zoro states he has something to look forward after King, implying a more interesting challange than the lead performer.
Both Zoro and Sanji could see what Queen and King could do more or less, they also discussed about "If we win this fight Luffy becoming PK seems more close".
They are not equals, like of course, what people debate about is basically "Is Zoro closer to Luffy" as you think or "Is him closer to Sanji" as I think.
I see Zoro and Sanji as having similar role in the crew, as Wings, as the ones taking the lead after Luffy, Vice Captain and Chief of Staff, but yeah Zoro has higher authority, stronger and is more important than Sanji in the story. But Oda basically has Sanji as the third in fighting, importance and authority in the crew and basically sees The Monster Trio as the Protagonists after the Time skip, making Sanji more relevant than Nami (but many will cope about that), Monster trio is the symbol of One Piece alongside mascotte Chopper.
