Okay, Zoro is stronger, either a little bit stronger or a lot stronger... This difference doesn't matter if in battles they defeat characters portrayed close in level. Sanji is several times more useful to the crew than Zoro. Sanji cleans the kitchen, does the shopping, he's the best cook in the world, he's a tactician, currently he must also be literally the best spy in the world. Zoro's job is to fight and it's over. That's why their rivalry isn't fair, Zoro will never be able to contribute that much. So I don't think Sanji should have received the title of wings, it was fairer to Zoro, Yamato being his rival and one of the king's wings, because Yamato also has ACOC and their role in the crew is extremely the same: being a combatant = being the wings of the PK.