When comparing Xamarin.Android vs RAD studio, the Slant community recommends Xamarin.Android for most people. In the question“What are the best IDEs for Android development?” Xamarin.Android is ranked 5th while RAD studio is ranked 8th. The most important reason people chose Xamarin.Android is:
Xamarin is a platform on which you can build cross-platform mobile applications for Android, iOS and Windows Mobile and use only one codebase.
Specs
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Pros

Pro Cross-platform
Xamarin is a platform on which you can build cross-platform mobile applications for Android, iOS and Windows Mobile and use only one codebase.
Pro Fast build
Faster compared to Gradle-based systems.
Pro Allows .NET programmers to write Android Apps
Allows .NET programmers to write Android Apps.
Pro Programming in C#
Allows you to write your programs in C#, a language much superior to Java, regarding expressiveness, readability and overall productivity.
Pro Open source
Xamarin.Android and the whole Xamarin SDK is free and open source and released under the MIT license.
Pro Latest and greatest cross platform compiler for delphi
Currently featuring iOS, Android and windows cross compiler.
Cons
Con Lacking third-party library support
Having to use third-party libraries in Xamarin can be a real pain. Since Xamarin uses C# and third-party libraries are written in Java, you have to create bindings to use them in a Xamarin app. Which is cumbersome and wastes a lot of time. Especially considering that the Xamarin docs are not very good when it comes to this part.
Con Too large to download
Since core Visual Studio comes about in 543 MB of download size, the Mobile App Development with C++ workload comes about ~4 GB, the issues with your bandwidth and internet connection can cause the components to re-download, wasting a considerable amount of internet.
Con Big price tag
With a price tag going between 2700$ to 6500$ for the latest version RX10
Con Subscription updates
If you want your bugs fixed, you will need to pay. Most other companies offer those for free.
