e.g., JDK debugging with hot-replace means you just fix errors, recompile, and continue the same debugging session. Run-everywhere means you share knowledge and libraries with 10-100X more people. Multi-language support and rapid iterations mean you get all the latest features.
Don't be confused by demo-ware that "just works". Most any domain is hard, and making initial steps easy usually means later you have to learn the real programming model underlying the simplicity-adapter layers, and work with multiple mental models.
That said, pick a domain and a community, and follow them, because they're unlikely to collectively pick the wrong platform.
Of course, I'm writing this from the perspective that I'm gonna be using Clojure, which is probably different than your situation. And without knowing more about your situation, it is short of impossible to know which one is the "right" choice.
So without knowing more, I'd say go with the ecosystem and language you're the most familiar with.
Unless your goal is to learn something new, then go with the ecosystem and language you know the least about already.
.NET is Microsoft environment only (no matter what Miguel may say).
Java is cross-platform.
As the old saying went: 'Why halve your chances at the cocktail party?'