https://developer.chrome.com/blog/wasmgc/
Kotlin's support is in Alpha right now, but works well, and it's one of the first language implementations that can target WasmGC:
https://blog.jetbrains.com/kotlin/2023/12/kotlin-for-webasse...
By comparison, .NET's Blazor targets LLVM, and they either AOT or JIT, however the client has to download a heavier runtime that has at least a garbage collector (nevermind the JIT), and is less than ideal. Basically, the original Wasm was designed for languages with linear memory and still makes a great target for C++ or Rust, but not for managed languages like Kotlin/Java. dotNET's WASM is there only to support Blazor, which is a web framework, a sort of successor to Web Forms and whose future is uncertain. Speaking of which, you're better off with MVC + HTMX, but I digress. So for more interesting use cases, Kotlin is actually ahead in their Wasm support.
For multi-platform support, the company behind it has a vested interest in targeting multiple platforms, and Kotlin Multi-platform support also has Google backing. So, for one, you can share business logic on iOS, as you can integrate Kotlin libraries into Swift applications:
https://kotlinlang.org/docs/multiplatform.html
And they have been porting Jetpack Compose to the desktop and to iOS (and can be used with Wasm in the browser, too):
https://github.com/JetBrains/compose-multiplatform
Also, see this year's keynote at KotlinConf:
https://www.youtube.com/watch?v=c4f4SCEYA5Q&t=2903s
Over on dotNET side, the blessed solution by Microsoft, for targeting iOS, Android, or the desktop, is right now .NET MAUI. So, where's Xamarin? Where's Silverlight for that matter? That's right, Microsoft changes multi-platform solutions like they change socks, and I don't understand how anyone could trust them for a multi-platform solution.