It really depends on what you plan to develop and the IDE you plan to use. Here are a few points worth mentioning:

• Kotlin is generally better for mobile application development. Most documentation you will find for android development will use Kotlin.
• C# and ASP.NET is better for web development
• C# is a Microsoft product so Visual Studio is better for C# programming
• JetBrains designed Kotlin so IntelliJ or Android Studio is better suited for Kotlin programming
• C# is more commonly used for Object Oriented Programming
• Kotlin has better support for Functional Programming
• Both support asynchronous programming, but I would argue Kotlin has better built-in support

Overall, both are great languages and have tons of applications. I picked Kotlin because it is more concise and efficient language, but that doesn't mean Kotlin programs will inherently be more efficient and performant. I would say if you are wanting to learn just for personal projects and such, Kotlin is probably the way to go. If you are learning to get a job as a developer, C# would be best. Once you learn the basics of programming, you will find that learning new languages is not difficult. So whichever you decide to go with, spend time understanding programming concepts more than just the language itself.