One has structures, ARC, proper extensions, good protocols/interfaces, flexible enums, and runs natively. The other one doesn't. I think I'll choose the former.
They're not really in direct competition. Try writing an Android app in Swift, or an iOS app in Kotlin. Technically possibly either way, but one's definitely more suited than the other for each purpose.
I honestly didn't know that I could use either Kotlin OR Swift for both platforms. But which would be easier to get running on Windows? I also need to write a version that works on the web, and I've heard that Kotlin can transpile to JS. So that's pretty amazing if that works.
Otherwise I was considering C, C++, or Rust.