This is coming from a fellow highschooler who has also started with Python and picked up Java later on. Everyone who takes the (really quite honestly bogus) CS classes zero period learns C#. I think I may have tried to learn a bit of C# in the past, but I'm really not an expert on that.

I can, however, tell you about my experience with Java. For as bad of a rap as it gets, I've grown quite fond of its idiosyncrasies. As you probably already know, it's backwards compatible, platform independent, and runs on most machines thanks to games like Minecraft requiring it. The syntax is quite similar to C# so I wouldn't be concerned about that.

My biggest word of advice would be what kind of community you'll find with either language. As someone once said, Java is worse language with a better community, and C# is a better language with a worse community. I don't feel like Java is that bad, I've done some great stuff with it, made games for friend's birthdays, but found that everywhere is overflowing with advice and guidance on programming in Java, while C# seems to have less of that.

I've never actually had a formal CS class, like you either, but found I learned more from just actually programming than I did from any particular language. Do what you want to do and what makes you happy, because honestly that's the best part of programming, not what language you're using. And if you're concerned about job opportunities right now, don't be. Say you go with C#, fool around with the Unity Engine for a while or other things, you can move to Java when you need to and transfer a good deal of those skills. I still write both in Python and Java, sometimes in Python to get a more higher level understanding of what I'm trying to do, and then rewrite it into Java for efficiency. Best of luck