Java has comparatively bigger market compared to .NET because .NET runs on only windows platform and Java runs on all the platform. Also .NET is comparatively older than Java.
While saying that both Java and .NET are neck to neck these days in terms of technology. .NET is so good with functional programming specially Lambda expression while Java implemented to those only on Java 8 onward.
Both of them rely heavily on OOPS. So if you know OOPS very well then I do not think you will have problem switching from one technology to other.