Scala is far more mature, as things stand
Kotlin has a lot of similarities with Scala, which has IMHO been far a better JVM language than Java for quite some time.
By far the best feature that Kotlin brings to the table is it's attempt to rid programming of nulls. Scala had an experimental feature (based on the NotNull marker trait) to do something similar, but it has since been deprecated due to some technical issues.
Otherwise, Scala is currently a lot more mature than Kotlin and has many more useful features than both Kotlin and Java. As for it's ease of use, Scala does have a bit of a steep learning curve, not least of which is re-training your brain to code using functional programming principles. However, it is in many ways a simpler language than Java, yet you can still write Java-esque code in Scala if that's what you're most comfortable with.
Otherwise, macros, implicit conversions, implicit arguments, traits/mixins, type inference, fully object-based type system (including primitives and functions), custom operators, ability to create DSLs, etc. etc. make Scala a boilerplate-free alternative to Java. No-one needs Kotlin as things stand.