Xamarin is basically done to create mobile cross-platform apps using all the mono core code. But as I know, this IDE permit you to do some of other type of codes (F#, C#, ASP…), so for a beginner this IDE is enough. When you will have more experience, using VS will be better (if Microsoft do not implement new things in Xamarin IDE in the future), because all the integration of the Microsoft environment is amazin in VS. But for now, with you missing space in your MAC, the easiest solution is Xamarin ! (Care when you will install the Android Package in Xamarin installation)
When it comes down to editors/IDE's, there are a number of cross-platform alternatives. Not all are equally well-equipped.
Visual Studio Code is the new and lightweight IDE from Microsoft. It runs on Electron Shell (basically Github Atom) but is not as powerful as full Visual Studio even though the naming may let it seem so. It is more a code editor with a little extra (like type checking and building).
Xamarin Studio is a quite capable IDE, but is less configurable than Visual Studio. Also, it is focused more on Xamarin development, and less on general C# development.
Jetbrains Rider (codename) is Jetbrains' proven ReSharper extension, wrapped in its proven IntelliJ IDE. It is still in development but will be probably quite similar to Visual Studio in terms of functionality. It won't be free though.
Also, bear in mind that there are multiple runtimes for the same language and that there are differences between the runtimes!
.NET (Full) Framework: this is the reference implementation from Microsoft but only runs on the Windows Platform
Core CLR: this is a Microsoft Supported Cross-platform runtime, but only contains core functionality. You cannot use all the libraries in the full framework on this CLR.
Mono: this is an open-source cross-platform implementation of the full-framework. Obviously, not everything can be ported to all platforms, so there are pieces missing. Also, not all libraries are available.
