Earlier quoted context omitted.
Considering Microsoft has no Mac IDE, I'm not sure how Xamarin could be considered a competitor.
...because Visual Studio is a C#/C++ IDE that works on Windows, and Xamarin Studio is also a C#/C++ IDE that works on Windows? Xamarin Studio is a swiss army knife. I don't know about your use case, but I use it under the name Monodevelop on linux (not really sure what that association is) and as Xamarin studio on windows. For my use case, it's a C# and C++ IDE for desktop development, same as visual studio. It's cer…
Mono only supports a subset of the whole stack.
No sane .NET developer would use Mono on Windows, its purpose is .NET portability to other OSes.