I currently use Gradle because I find it simpler to specify the build files with Gradle than Maven.
However I see it as a matter of personal preference and I don’t think that there’s an actual “preferred way”.
Is the project personal, or for company? What else is used in the company, or can you set the standard? How big will the project(s) become? How many modules and sub-projects might there be?
What are you most familiar with? Do you want to learn a new build-tool alongside learning Kotlin?
What IDE do you use, and how well does the build system integrate in your IDE? (NetBeans has best Maven integration of any IDE I’ve seen but worse Gradle integration, for instance).