C# is a much better language and has better stewardship but Java has extremely broad usage.

Plus, speaking from personal experience, learning Java first and then learning C# is probably going to be a positive experience, whereas doing it the other way around feels like going back ten years in time.

Way more mature for linux, with all Microsoft efforts, C# still has a long way to catch up on that platform. Also, your Java skills will be transferable to Android.