Both Zoro and Sanji lead in their own very different ways, it’s a phenomenal dichotomy that Oda has to be doing on purpose.
Recently I compared these two to the Guardian Angel and the Demon of the Straw Hat Pirates; and I believe that is a very good comparison. Most of Sanji’s duels begin with him protecting others. In Alabasta he fought Mr. 2 to protect Vivi. In Enies Lobby he fought Jabra to protect Usopp. In Thriller Bark he fought Absalom to protect Nami. In Punk Hazard he fought Vergo to protect Tashigi. This is a trend Oda created to highlight that protection is most important to Sanji.
The same recently happened with Sanji rescuing Momonosuke using his Raid Suit after King thwarted Shinobu’s attempt to do so.
Now contrast it with our Demon Zoro. Zoro’s fights have the goal of victory in his mind. When he fights all he’s considering is how to win and grow stronger. And his one liners during the beginning of duels showcase that ideology of power and victory. Against Mr. 1, “I’ve been waiting to be backed into a corner like this. When I win, I’ll be able to cut steel.” Against Kaku, “It’s moaning, it wants to taste blood.” Even against Ryuma, “When I win, I’ll make that sword mine.” Just like the Asura’s of mythos, Zoro acts like a power hungry demon to be feared.
Protection and Power, neither is superior to one another; and this goes along well with the way they work in the Straw Hat Pirates and the leadership roles they take.
Sanji is a man of thought and acts very akin to a teacher; again look at how he talks to Usopp in this scene. He doesn’t tell Usopp to go and snipe, but rather to assess the situation and figure what he can do. This leads to Usopp figuring out for himself what he can do as well as help him realize his own importance in the crew.
Now Zoro on the other hand; he’s a man of action. He is the first one to enter a bout and this recklessness and surety inspires others to follow suit. They see this man lack a fear of death, and they can’t help but emulate this. Look at the Oars fight; or the Birdcage. Both times, Zoro rushed in first and then others followed suit.
Now, lets combine these and hopefully I can explain how this affects why I call them Quartermaster and Right Hand, Guardian Angel and Demon, and Shield and Sword of the Straw Hats.
The Right Hand of the Captain is a part of him. Just as Zoro and Luffy have a special bond that is different than every other bond in the Straw Hats. Zoro trusts his captain to be strong enough to do what is necessary and as long as he is; he’ll continue to ensure that Luffy’s path is clear, while simultaneously making sure that Luffy is prepared for what must be done. Whenever Zoro showcases his leadership, it is directed at Luffy. His speech to not allow Usopp back without an apology, his comment after Luffy’s fight with Usopp, and his comment when Luffy was preparing his rematch against Caesar; were all directed to get his captain’s head out of his ass. Zoro holds Luffy to a standard and ensures that Luffy rises to that standard. That is the perfect right hand to me. The man that makes sure the captain is a captain to put faith in.
Compared to Sanji who typically directs his leadership to the crew unless he’s sticking up for a crewmate. When Luffy was about to tell Usopp to leave, Sanji stuck up for the crew. When Zoro was talking to Luffy about Usopp not rejoining, Sanji directed his comment to Nami and Chopper, when he was reminding Usopp of what he can do. Oda has written it such that Sanji stands up for and directs the crew; while Zoro stands up to and raises up Luffy. Quartermaster and Right Hand.
Now let’s consider Dressrosa, which is another example of these two showcasing themselves as the angel and demon. Pica is literally an obstacle in the way of Luffy’s path to Doflamingo. He was a giant wall, again quite literally. So, what did Zoro do? He got Luffy over the wall, and insured it never blocked his path again. Now, compared to what Sanji did; he was about to sacrifice himself to Doflamingo to ensure the crew’s survival. That’s what they do. It’s who they are. Zoro is always slicing down objects in Luffy’s way, sometimes literally like the sea train that was in rocket mans way on the way to enies lobby, while Sanji is sticking up for the weaker members of the crew to protect them.