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