.NET and Java are very-closely related, and it's almost foolish to pin one against the other, but if I had to pick one, I'd definitely side with .NET.
Why?
1) Garbage Collection in .NET - much better than Java.
2) Arguably the best editor on the planet, Visual Studio doesn't support Java... but coincidentally, it DOES support the main set of .NET languages. Funny how that worked out, heh?
3) .NET supports different languages, and not just the main set of languages that everyone thinks of like C# and VB. There's also J#, IronPython, and someone even ported COBOL and FORTRAN to .NET. Can Java do that?
I'm at work, so those are the first three that come to mind! Come on, Java camp - can anyone do better than ".NET is for sissies?"