It depends on your specific circumstances.
I do not pretend to be the exact answer because it does not exist, but here are some of my thoughts and feelings. (Upfront, I'm primarily a .NET developer, but I really work in Java for a while)
: .NET, , 80% , . Java , , .
, Java , # Windows. Mono # Linux Mac. , iPhone (MonoTouch) Android (MonoDroid).
, # - , Java . , - ++...
Visual Studio , Eclipse. , Visual Studio, Eclipse . VS , , MS. Eclipse .
J2EE , . , .
.NET Windows IIS/WAS/(Server AppFabric), , , , , .