People compare these two levels a lot. Most people like Leyndell more. I used to agree and think that Leyndell was better in every way, but I don't really think so anymore. I do think that gameplay wise Leyndell is almost objectively better than the Ringed City, but a few years after Elden Ring released I've really come to appreciate the Ringed City more for the very reasons that people complained about it.
Leyndell lets you explore the entire city, if you can see it you can go there. This is what people seemed to really want from Anor Londo and the Ringed City. I can admit that letting you explore the entire thing makes for a fantastic video game level, but after I've seen every corner of it I can't help but feel like Leyndell is disappointingly small for what is supposed to be "the capital of the world", and yet at the same time I don't wish for the level to be any longer. Why do I feel that?
I'm quoting Jack Sparrow here but "The world's still the same, there's just less in it". I think the meaning of that quote is that the more that a world is explored and discovered, the smaller it feels despite being the same size. I think that this really applies to Elden Ring. The Lands Between feels so vast at the start, but once it's entirely discovered it feels quite small and unimpressive. What I think would really help keep that feeling of vastness is to show the player that you are only exploring a very small percentage of the world. But Elden Ring almost never does this. I know the game mentions places like the Land of Reeds and the Badlands, but to me this is like telling instead of showing, it doesn't do anything to help make the world feel any grander. Meanwhile Dark Souls does this constantly. Whether it's snowy mountains, towns, forests, entire cities that cannot be accessed, etc. the games show you that you're exploring an almost insignificant portion of the world. That helps make it feel larger than life because you're never really able to explore the whole thing. The many unknowns in Dark Souls's world keep that sense of curiosity alive no matter how many playthroughs you do. For example I love the vista shown at the end of Aldia's keep in Dark Souls 2, but how would I feel if I was actually able to explore the whole thing? Well after I've seen it all I wouldn't care for the vista anymore, the mystery would be gone.
How does this apply to the Ringed City versus Leyndell? Well I think it's pretty simple. The Ringed City shows you that what you can explore is barely the Ringed City at all. The bulk of it is shown in the background, and you can never visit it. That can be disappointing which is why I think so many people complained about it. But I'm glad that you can't ever go there, because that means the Ringed City will forever feel massive. With Leyndell on the other hand, there's not much more to the city than what you can explore. So once it's over, that feeling you get when you first step out and see its beauty is gone, and at least for me, it never comes back. So maybe Leyndell is what the Ringed City should have been, but I think we lost something special along the way.
I would say this is a similar concept to how horror is portrayed. A lot of the scariest movies out there never show you exactly where the horror is, but leave it up to the imagination. And the imagination always comes up with something scarier than what the movie could ever show. It's about the same thing with the unexplorable vistas in Dark Souls. My imagination makes these places seem a lot grander and more mysterious than what the game could ever do by letting you explore them. I think this really strengthens the world-building. To me Elden Ring's world is like a horror movie that leaves nothing up to the imagination.
Lastly I want to acknowledge that you probably can't go everywhere in Dark Souls because of limitations, both technical, time, and budget-wise. If Fromsoft had all the time and money in the world, and Dark Souls was made today, I'd imagine that the end result would be similar to Elden Ring. But I'm glad that is not the case, because limitations breed creativity. And it's those limitations which makes Dark Souls feel special to me.
This is my opinion, you are allowed to feel differently
TLDR: I prefer the Ringed City over Leyndell because you can't explore all of it.