angular 6 dependency injection

Basically you can use either, But as per new CLI provideIn will be automatically added while creating service

#providedIn

There is now a new, recommended, way to register a provider, directly
inside the @Injectable() decorator, using the new providedIn
attribute. It accepts 'root' as a value or any module of your
application. When you use 'root', your injectable will be registered
as a singleton in the application, and you don’t need to add it to the
providers of the root module. Similarly, if you use providedIn: UsersModule,
the injectable is registered as a provider of the
UsersModule without adding it to the providers of the module.

This new way has been introduced to have a better tree-shaking in the
application. Currently a service added to the providers of a module
will end up in the final bundle, even if it is not used in the
application, which is a bit sad.

For more information please refer here

• https://blog.ninja-squad.com/2018/05/04/what-is-new-angular-6/
• https://angular.io/guide/dependency-injection#injectable-ngmodule-or-component
• https://angular.io/guide/hierarchical-dependency-injection#moduleinjector [As suggested by Tuan-Tu in comment below]