From my experience of working with both Java and .NET I can say that: the weakest part of the .NET technology stack is community, and the strongest part of the Java world is community which produced the huge pool of quality, free and open-source frameworks. Currently Maven repositories count for more than 14,000 unique artifacts and almost 60,000 versions (http://www.mvnbrowser.com/faq.html).

C# is a nice language, but when you have to develop a real-world application, it does not help much. .NET simply lacks too many essential tools and frameworks. You can do basic things with it, but when you have a non-standard task at hand, you are alone to implement the solution by yourself or pay for crappy third-party closed-source component. Even for trivial tasks. Want a decent HTML parser to sanitize HTML documents? Write it by yourself or pay for crappy third-party closed-source component. Want to read email messages from the server? ... you got the idea. The list goes on and on. I am currently working on a C# project which has integration with source control systems. We had to invest a lot of time to write our own framework and providers for several most widely adopted tools, such as Subversion and Perforce. If we based our project on Java, we would saved lot of time on development by choosing Maven SCM providers, which are free and open-source. Another example is that we needed a decent audit history for the documents in the database. Again, we had to develop somewhat working home-grown solution. If we based our project on Java, we would simply pick open-source Envers framework (http://www.jboss.org/envers/) which does exactly what we need for free.

And I hate Visual Studio. It offers a little more than a notepad with syntax coloring and debugger. It is horrible for coding and those variegated wizards and diagramming tools don't make me productive either. Being unable to attach third-party source codes is a huge PITA. Eclipse in size is just a fraction of the VS installer, but still is feature-packed, better for refactoring and has a plenty of plugins. Speaking of IntelliJ Idea, it's simply the best IDE ever.

.NET build and CI tools suck, there is nothing to compare with Maven.

If you like to learn new languages, I think there is nothing to beat JVM in terms of variety programming languages running on top of it. Just for example, the most notable ones are JavaScript, Python, Ruby, Groovy (Python-like language with Java-friendly syntax), Scala, Clojure (Lisp-like), CAL Language (Haskell-like) and many more. By the way, JVM by itself is quite a decent piece of engineering.

My experience makes me think that Microsoft is trying to be popular at any cost, targeting inexperienced programmers, intentionally oversimplifying their solutions, making simple things trivial, complex things impossible. The Microsoft culture is to give you a big bright flashy button, which does a lot of things under the hood, but when accidentally it breaks, you are unable to guess what is going on and how to fix it. Living in the world of closed-source .NET code makes me cry.

So the conclusion is: choose Java over .NET, but learn more uncommon programming languages as well. Haskell, Lisp and Prolog were huge "brain-washers" for me.