If it's only android kotlin is better.

If eventually you need an iOS app then use flutter.

Kotlin you get you a better performance if you need to do something heavy.

Flutter works for apps that don't need much processing power.

In my work I have a flutter app with 200+ screens but the app only do get, post, delete requests and take some pictures.

If you need some background processing, notifications and other stuff using kotlin is a lot better