I'll give you a broad answer to your question:
C# is a streamlined platform that is designed to work with Microsoft's platforms and tools. If you use what Microsoft has to offer (Visual Studio, Windows, NuGet, IIS, ASP.NET MVC) and some of the really well-maintained OSS packages, you'll have a great experience. Bad news is: you don't get a lot of freedom in terms of platform choice.
Java is a broad platform that comes in thousands of different flavors; there are tons of mature, well-maintained options for just about everything from web framework to IDE to build tool and so forth. The problem is: everyone does everything their own way, so you kind of have to jerryrig everything yourself. Java can feel really janky, but you have a ton of options.
disclosure: I'm ex-Microsoft and LOVE .NET, but I feel that I need to know the JVM better in order to work on some of the projects I love.