According to last 5-6 year.NET is much better than JAVA. Technically speaking it is not a standard set by the industry. It is an evolutionary facet of skills. A full-stack developer always tries to learn open source technologies to apply them in his/her job. In my opinion, full-stack development is a minimalistic combination of the following: Front-end Development + Back-end Development + DevOps + Design.
It's something like:
1. Java very similar to C#.
2. For web C# with .Net much better than Java.
3. Java for android much better than Xamarin.
4. Java more preferable for secured websites.
5. .Net more preferable for platforms with ms integrations(Azure, ML and so on).
So you should decide in which area you will use this language and choose one that more preferable.