A startup should choose .NET right now. With the things that are available in .NET and especially after going opensource, .NET is the framework for building all kinds of application. With the help of Xamarin, you can easily build mobile apps, with the help of WPF, you can easily build desktop apps, and with the help of ASP.NET and WebApi, you can build all your Web apps. All of these is possible with the best IDE in the world i.e. Visual Studio and only one language , i.e. C#. So a start up can have one man army do everything for them(As long as that person has mastered the concepts of .NET Framework and C#). No kidding!!!