[pkolaczk]

> Scala adds like 500kb to an Android app

This is a lie. Please stop spreading these lies about Scala requiring a huge runtime. Scala runtime adds about 30 kB, which is nothing in 2017.

> the tooling support is also better.

Both use the same IDE from Jetbrains and both are officially supported. Hard to tell which one is really better, but Kotlin support has more issues in its bug tracker, despite being less popular :P

> (nulls) it is very well done.

It is not. It makes the type system more complex, and you can't abstract over optionals and other collection types. You also can't make your own flavor of optional-kinds, like Either or Try that would behave similar to the builtin nullability feature.

The biggest problem with builtin features is that you can't make your own slightly different ones. You're at mercy of one company controlling the language. With a smaller language + libraries, you can always fork a library and do modifications.