Honestly, it depends.
If you're with a C# shop, it makes more sense to work with C# so you can collaborate with the developers and share code. Similarly, if you're working with a Java shop, go with Java.
A few other considerations:
- The main tool for C# development is Visual Studio. While there is a free version, the licensing for it is quite limited.
- There are a number of free tools for Java development. Eclipse is one of the better known ones.
- Selenium plugins and extensions differ depending on language. I don't know which of the languages Selenium supports has most support: that may impact which way you choose.
Other than that, C# and Java are very similar in terms of what they do and how they do it. I doubt it will be difficult for you to switch language if you need to.