When comparing Xamarin.Android vs Qt Creator, 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 Qt Creator is ranked 9th. 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 Great syntax highlighting and auto-completion
Qt Creator has a code model which basically has the same information as the compiler. So it can do really nice syntax highlighting (e.g. of virtual methods or local variables) as well as provide great code completion.
Pro Integrates well with non-IDE workflows
Qt Creator uses normal .pro-files, CMakeLists.txt, Makefiles.am, etc. for its projects and rarely needs special configuration for projects.
Projects can be built on the command line as usual.
Pro Built-in Qt GUI editor
Allows for the creation of a window based UI in a graphical editor, no code required to build the UI.
Pro Fast and fully keyboard-navigatable
Responsive UI, no need to use the mouse for the power users.
Pro Supports CMake
Pro Very responsive when compared to similar software
Pro Much space dedicated to the code
Small and beautiful UI, almost all the space is dedicated to the text with hardly and toolbars. Can actually be used on a 1024x768 pixel screen.
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 Poor refactoring
QtCreator has lack of refactoring features. It's not even close to Resharper++ or CLion.

Con Poor multi-window mode support
While multiple windows are supported, many operations will activate in the primary window (debug, goto-line... etc).

Con Qt-focused
Qt Creator is focused on being an IDE for Qt, as a general purpose IDE it performs quite well, but there are areas which are lacking such as project file support (support for generic/CMake projects lags behind Qt projects).
