CHARACTERS AND THEIR AMBITIONS
Zoro is a man who wants to become the strongest swordsman in the world and Sanji wants to find All-Blue.
In Zoro’s case, power is INTEGRAL part of his dream. Whenever we see Strawhats taking their sweet time, Zoro is either sleeping, drinking or training, and this is very important. On the other hand, fighting-wise, I believe Sanji is more of a spy-type fighter, a rogue if you could call him that, and his specialty in fighting is getting the task at hand completed at minimal risks and in the smartest way possible. He is the Cipher Pol of Straw Hat’s World Government.
Pre-timeskip
The best way of comparison of their powerlevels pre-timeskip, in my opinion, is the powerlevel ranking system of the CP9. I believe that the monster trio of Straw Hats somewhat resembles the monster trio of CP9, having Luffy at top, Zoro 2nd and Sanji in a veeeeeeeeery very close third spot.
Post Timeskip
I feel like this shouldn’t even be a discussion. Zoro spent 2 years just training his combat skills, while Sanji was actually working on his specialty – cooking. He spent a lot of time running and improving his legs, but this doesn’t lead to him being generally stronger, especially in Armament Haki area.
Oda deliberately made Sanji fight Vergo and Zoro fight Pica, and I think this is why: Sanji is good at Observation Haki but not good at Armament Haki and thus he had to fight an opponent who outclassed him in his weakness. On the other hand, Zoro was good at Armament Haki but wasn’t good at Observation Haki and had very much trouble finding Pica in the stones.
I think that Oda deliberately did this to show their weaknesses, and by all things natural, I think that having better Armament Haki is more crucial to being a better fighter in One Piece world, as shown by Luffy beating Enel when Luffy didn’t even have any Haki.
*Theory by MarineRitter