Zoro always has shown the qualities of being a loyal crewmate of Luffy and is perfect First mate.
Strengthwise no one doubts is capability as a leader and would make a strong Captain whose name can renown in parts of Grandline. Zoro also has strong will and ambition that he wants to follow with everything, which has been shown to be a great quality in One piece.
And even tho he doesn't have many examples of people following him, in DR recently he inspired everyone to try stopping the Birdcage with him. In Wano Yasu was influenced to follow him arround.
On the other hand Sanji is a cook and always has handled men under him. And we recently saw him takin care of half of SHs during his journey from DR to Zou, and he did his job well of saving his crewmates and heading on to take care of business himself, kinda like what Luffy did when he took Katakuri with himself in the Mirror world.
Sanji has also shown the qualities of makin allies fast, just like Luffy which Mihawk mentions in is one of deadliest abilities on these Seas. Sanji did it first with Gin, and last with Pudding who was told to kill Sanji but wants to help him n his crew instead. Sanji also made Duval n his members follow him even tho they were enemies at first.
I am sure in both cases there are more examples of their leadership qualities, i wrote em here top of my head. And both will make good Captains.
Bt if i have to name one as better, i think Sanji is more well round leader out of two.