I am looking for some reliable statistics regarding Java vs. .NET marketshare. I am interested in Java as a platform (application servers etc.) not as a language, so this should include Grails/Groovy, JRuby etc. in Java camp and C#, VB .NET etc. in .NET camp.
Take a look at TIOBE index: http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html
What is TIOBE index: http://en.wikipedia.org/wiki/Tiobe_index
It doesn't really matter. Both are large enough that you'll have no issue finding developers to work on it no matter which platform the product is running on.
It's like asking which company, Ford or Chevy has a bigger market share because you're going to buy a new truck. It doesn't matter.
.NET is if you want to use Windows as your platform forever. Java is if you want to postpone that decision.
Please note that besides that I consider hé two roughly equal.
If you are talking not just about the "language" but the entire platform, than you should consider the mobile phone market too - the majority runs Java (much bigger than the entire PC base), and the SmartCard market too (also runs Java).
Than there's the question what you really like: something controlled by only one company (in the case of .NET), or something that is driven by specifications and an entire industry behind it?
Just similar to Tiobe index, you can have a look at http://stackoverflow.com/tags , you can see c# is at no. 1 position consistently since long. Both are equally large enough, but I see new things faster in c# compared to java.