I don't think the number of Stack Overflow questions is a good indication. Given the difference in age and Scala's use of features unfamiliar to many programmer

I don't think the number of Stack Overflow questions is a good indication. Given the difference in age and Scala's use of features unfamiliar to many programmers I'd expect it to generate more questions from (for example) people switching from Java, especially since those unfamiliar features are probably only known to people like Haskell developers who are rare bunch themselves. Kotlin being a bit more of a "Java done right", a bit less radical, maybe generates less confusion. It might also be a sign of Kotlin's documentation being better. I thought the article was pretty weak, it doesn't really go into much detail into how things like higher-kinded types actually affect Scala programming and why you might miss them (or not) in Kotlin, but does take the time to note that Kotlin needs an extra keyword on the constructor arguments for data classes, something which is almost completely irrelevant (and, IIRC, comes from Kotlin's primary constructor syntax so is a reuse of a more widely applicable pattern). I don't think I could come away from this with an idea of which one I should learn and use, as it claims.