I don't do Swift or JS/React professionally (well, tiny bit, but I'm not a "React professional"). However, I've made non-toy Swift/SwiftUI* (~7k loc) and React (~4k loc) project.
Imho Swift/SwiftUI is way more of a joy to work with. In particular, SwiftUI is much easier to visually parse whereas JSX is noisy and IMO hideous. I also ended up needing to bring in way more third-party dependencies for JS/React, something I hate doing if avoidable**.
* It is my understanding that many iOS professionals prefer UIKit bc SwiftUI is still missing stuff. I only ended up needing to fall back to it once or twice.
** ClojureScript/Reagent do solve most of my issues with JS/React (by essentially replacing them). Clojure is well-designed with a great standard library (so less need for dependencies or implementing stuff that should be in a standard library). Hiccup is way preferable to JSX.
Imho Swift/SwiftUI is way more of a joy to work with. In particular, SwiftUI is much easier to visually parse whereas JSX is noisy and IMO hideous. I also ended up needing to bring in way more third-party dependencies for JS/React, something I hate doing if avoidable**.
* It is my understanding that many iOS professionals prefer UIKit bc SwiftUI is still missing stuff. I only ended up needing to fall back to it once or twice.
** ClojureScript/Reagent do solve most of my issues with JS/React (by essentially replacing them). Clojure is well-designed with a great standard library (so less need for dependencies or implementing stuff that should be in a standard library). Hiccup is way preferable to JSX.