<p>I haven’t been up to date on the programming field for quite awhile, so I’m not sure how useful C# is compared to Java. But it is good enough to just learn one of them, unless you are becoming a software engineer. SEs will learn more than one for a wide variety of reasons, but if you are in any other field or you just want a hobby or even a skill, you could just pick one, focus on it, and be fine. Then later you can learn more that will be suitable for further reasons.</p>
<p>Java is very good to learn. It’s widely used and serves a wide variety of functions in software development. You can build almost anything with Java.</p>
<p>I don’t think C# is interchangeable with Java. They are very similar, but you will still have to learn and deal with the differences. And I don’t even know if there are well working versions of .Net for many of the mobile devices such as iOS, Android, Symbian, etc. What if you wanted a program to run on an iPad, Android device, Windows Phone 7, or other devices?</p>