Luffy and Zoro's leadership styles are similar, which is why they often get each other more than the other SHs do. Essentially they are action first type leaders, charging in head first which others then follow. Zoro's essentially the right hand to Luffy, when he puts his foot down it's usually in relation to Luffy as the Captain, meanwhile, Sanji is more of the rep for the rest of the SHs, where he voices and considers the needs of the others and not just Luffy's. It's why Sanji is more suited to being a protector of the weakling trio.