On a .NET/Java shop and we have like 50% of both stacks in every, single, project.
Nowadays I tend to spend most of my time on the .NET side, because I get to enjoy Valhalla, Vectors, async/await, TPL, since years now, instead of waiting for what might never come.
I get to be on both sides of the coin, which would be advisable to the Java team to better know the competition.
As for competition, we follow what's going on in .NET, JavaScript, Python, Go, and even far less popular languages such as Haskell and Erlang. But regardless of your personal preferences -- some share them while for others Java's superiority in GC, monitoring, compilation, backward compatibility, and language simplicity matter more -- C#/.NET is not yet in the same club as C, Python, JS, and Java in terms of longevity and popularity, and that's what I'm talking about here.
C# is more than 20 years old, it runs on everything and everywhere. It has multiple toolchains and IDEs