I would say Zoro is a bit stronger than Sanji, but that's as far as I would be willing to go when it comes to power level talk. I think there needs to be a more important discussion on how Sanji and Zoro have been portrayed, which can be more subjective.
Before the time skip, I liked Sanji and Zoro equally for different reasons. Sanji had some great moments by using his brain to get the crew through certain parts. The Mr. Prince stuff in Alabasta allowed him to go in and save the crew, which was vital. In Water Seven he got on the train and basically saved Robin until Blueno showed his ability. He was in some pretty silly but cool fights as well, and his speech to Usopp on Enes Lobby was pretty great. Zoro was more of a fighter than using his brain, but there was silliness in some of his fights (like how he tried fighting while Usopp was cuffed to him). More to it, his fights were great because he came through after taking so much damage. He was able to come out on top even after the beatings he received.
Flash forward to the last few arcs, Zoro has been a bore for the most part. Sure, his feats are impressive such as taking on an Admiral and cutting the giant Pica, but the impact isn't there. When Zoro cut Mr. 1, it was great because of the lead up where he took damage and through will finally pulled through. The Kaku fight was great because of the hilarity of the fight and what he went through. Same for the Thriller Bark scene, where he took even more on top of what he had. Now, he just breezes through fights with little effort. Monet being scared is cool in a way, but it lacked impact. Perhaps if Tashigi had won the fight, then she froze by Zoro's aura when trying to attack from behind would be cool. Zoro cutting Pica lacked impact because while visually cool, he only had trouble trying to hit Pica. On a one on one fight, Zoro was easily above Pica. Zoro had a certain smug to him before, but it was okay when his feats came with how much effort he had to put forward. Now he just smiles and wins with no effort, which is boring. Fishman island I let slide because of the placement, but he needs to actually be challenged soon.
Now lets look at Sanji. His win record is not so great, but what he has actually done his great and stands out much more than Zoro. In Punk Hazard, he retrieved Kinemons torso in Nami's body in freezing cold water. After that, he faced Vergo, who was pretty strong and was clashing equally until he got his shin broken. Even after that, he was running with no problem, jumping around and kicking Marines to safety. That right there is pretty cool in my book, as a handicap like that didn't effect him. Then in Dressrosa, he got Violet to get on their side which was a big help in some ways. After that, he went and attacked Doflamingo head on, who is way above his officers. He may not have scratched Doflamingo, but Doflamingo was impressed non the less. Then after all that, he was ready to face Big Mom's ship head on with the weaker members with him, and they came out alive and on something even crazier. The stuff Sanji has done sticks more in my mind because despite some set backs, he continues forward. Screw him not winning any big fights, that isn't a big concern compared to how they contribute overall in the end.
One Piece to me has always been about characters coming over odds and going in situations they may not be able to win. Sanji has been in a number of those but has come out okay. Zoro has been breezing through things, and only some humorous moments have made him stand out currently. Zoro may seem stronger than Sanji because of wins currently, but Sanji >>>> Zoro when it comes to individual moments and being an actual character for me.
