I think Zoro has more raw strength but Luffy has better techniques and haki
Zoro is a very bad match-up for Luffy, his offense and defense are way too good to be downgraded to Luffy's.
Zoro can end Luffy with one clean hit while Luffy needs countless hits to end Zoro, complete mismatch, that's why he is on his side.
Zoro is supposed to be Luffy's protector, to shield him from things Luffy cannot do, I thought it was clear after the rooftop...
And let's not forget Zoro's mindset - to be the strongest, that's why a conflict between Zoro and Kaido, because he is called the strongest in Zoro's presence. Zoro is the personification of strength and combat, Luffy is something entirely else, his strengths lie elsewhere.
I know Oda has fooled a lot of people and made them think that Luffy is stronger but that's simply Oda, he likes to play with people.