Well the Zou Arc is now over, a 6-month, 21-chapter fling on an elephant's back where we learned tons of new information about the outside world. Now that it's ended, let's compare it with its very different predecessor: Dressrosa, a 3.5 year, 102-chapter campaign that was for the most part very self-contained, with world-building repercussions only happening at the end. So I might as well ask: which one was better?
Let me give my answer: I like both, but Dressrosa was better.
Now, I'll explain why. The first thing I must address is that the formatting of these two arcs is as different as can be. Dressrosa was a huge, plot-heavy arc that marked one of the series' biggest milestones, while Zou turned out to be a transitory arc, setting up the Straw Hats' mission for the future. So you can't really grade the two on the same point scale. Ultimately, this boils down to opinion, and my opinion is that I enjoyed Dressrosa more than I did Zou, after both are said and done.
Zou is a great arc, don't get me wrong. One factor that I felt was an improvement over Dressrosa was its character development, as the minks showed themselves to be the anti-Dressrosa citizens, who were undoubtedly incredibly annoying, as they were not afraid to step up and take matters into their own hands. The minks who interacted with the Straw Hats showed incredible depth to their character that makes them eager to see them again, whereas I couldn't care less about seeing Rebecca again at Reverie. I suppose Zou benefited from this because all its character arcs were basically intertwined, whereas Dressrosa had to balance very different character arcs (Straw Hats/Law/Riku Family/Marines/Colosseum gladiators/Samurai), though I think it did so moderately well. The conciseness of the story benefited Zou a lot.
However, the action/adventure aspect on Dressrosa was not to be beaten. Not everyone cares about how much action is in an arc, but I do, and while it won't make or break an arc, it will affect my enjoyment of it. After a couple chapters of intrigue, Zou simply became a tour led by Wanda. No explanations were withheld, and the Straw Hats were never really required to do much. There were a few wrenches thrown in, like the samurai confusion and Jack's return, but both those things were resolved one chapter later. Compare that to Dressrosa, where the Straw Hats had to use their intellect and sometimes brute force to understand their quest and their destination. Which is kind of a shame, because Dressrosa wasn't NEARLY as interesting an island as Zou was. Each groundbreaking reveal wasn't told to the Straw Hats up front, it was given out in a timely manner that made each wait worthwhile. And the Straw Hats did so many different things on Dressrosa, from free-for-all matches, to undercover missions, to wild rides up a mountain, things were repeatedly shaken up. We labored along with the Straw Hats until every last stone was uncovered, every character arc was resolved, and we got a sweet conclusion.
Ultimately, I think Dressrosa and Zou are meant to be read two different ways. Dressrosa is this great big and huge adventure that is best when read all at once and then you can go back and understand parts you missed. Zou, meanwhile, seems like a consolation for the weekly readers' woes, in which we're treated to new information each release. As a weekly reader, I sometimes got fatigued with Dressrosa, and reveled in Zou's overthrowing goblet of information. Now that both arcs are over, however, it's really Dressrosa that stands apart. I've gone back and read Dressrosa repeatedly, reliving my favorite scenes such as the confrontation on Green Bit, Usopp freeing all the toys, and the entire ten chapters of Luffy's and Doflamingo's fight. Even parts that I hated when reading weekly, like Luffy recharging his Haki, flow much better when read as a whole. Meanwhile, Zou...now that I've read all the big reveals, what's the point of rereading a giant tour that just rehashes information? I mostly only reread Zou just to get research information. I suppose my favorite part of Zou to reread would be the flashback of Jack vs. the minks, which was the most action we got in the arc.
So tl;dr Dressrosa is a better collective arc while Zou is a better weekly arc. The two arcs really are as different as can be, so it's tough comparing them, but really in terms of memories and experience I will go with Dressrosa over Zou as my favorite New World arc. It just had more story features that appealed to me, and had enough strength to stand out on its own, character arc issues aside. Zou was never much of a real arc, just setup for the upcoming arcs on Whole Cake Island and Wano Country, which I am looking forward to immensely.
Do you think Dressrosa or Zou is better, and what were the merits/differences of each arc that stood out to you?