It’s such a childish thing to claim. C# is packed with features that Java doesn’t have, such as LINQ, (proper) unsafe code and asynchronous methods. C# also got WPF, whilst Java is still waiting on a half-decent version of JavaFX. Lets not forget that Java is getting lambdas after C#, and is copying the syntax.
But does anyone really care?
Again, if you just did research you’d know this isn’t true.
Mono is pretty stable on Linux, and has never been threatened by MS, ever. Microsoft have even standardized C# and parts of .NET as ECMA standards, released under licenses which make it perfectly legal to implement them, which in turn helps Mono development. They did not have to do these things. MS even helped to implement Moonlight.
Personally, if your building for Windows, then use C#, especially if your building an application because WPF is truly brilliant. If you building for non-Windows, then use Java. With XNA you can also target Silverlight, as well as XBox and PC. This will mean you could also build your game as a Metro app, and release it in the Windows store.
One of the main issues with Java is around the game libraries. Only a couple (LWJGL and JMonkey) are actively developed, the rest either get occasional updates (like JOGL) or are simply old and left to die (Java 3D). On the flip side, game development with C# is far more active, and has far bigger ecosystems. For example there are more books on using XNA, then there are game development books on Java. You’ll find more tutorials, frameworks, communities, libraries and games relating to C# then Java.
But ultimately my advice would be to 1) work out what exactly it is you want to build, and 2) try them both out for yourself.