If you are doing this with ANTLR I would advise you to use Java.
Terrance Parr[
^] who is the head behind ANTLR developed it in Java and the C# ports of ANTLR are managed by
Sam Harwell[
^].
A point that would speak for C# though is that there are now Visual Studio extensions available with syntax highlighting for grammars etc.. So you should probably go with what your peers are using. If your company does mainly Java related development stick with that else go for C#.
Regards,
—MRB