From a technological point of view, the two languages are similar. In fact, because of the success of Java, the C # API itself was designed to make it easy for Java programmers to learn C # with ease.
If your specification requires C #, use it. If you are choosing a language to learn, I recommend Java for the following reasons:
Free compiler and tools.
The best IDEs are free too (Eclipse, Netbeans).
The main application servers are also free (eg Tomcat).
It is a versatile language that you can develop applications for PC, Web, embedded systems, tablets and Android phones.