One is a bushido swordsman who has already clashed with Zoro in the past
The other is a womanizing dirtbag who hasn’t eaten food in over 3 years, and his nature powers juxtapose Sanji’s genetically-enhanced body (science vs nature)
They’re the second and third strongest members of the Marines (Kizaru will likely be out of the story by the end of Egghead), so it’s only natural that Luffy’s Wings fight the top 2 henchmen while Luffy takes on the leader of the Marines
Why Sanji?
In shonen, if there is someone at disadvantage, it is always the protagonist' side.
Sanji's element is fire, Greenbull is plant, fire > plant so it can't work like that.
Going by element, plant > water, thus the one who will fight Greenbull is Jinbe.
Ignoring the fasting vs cook bullshit, Jinbe vs Greenbull makes more sense morality wise.
One is against the racism, while the other licks CD's dick who are very racist.
One was working for Government and realized it is not worth it, and the other is actually being happy sucking Government's dick.
It is Jinbe vs Greenbull.