It all depends on your goals, your learning style and the content of the classes.
If you would like some hands-on experience that you can apply immediately, I would go for advanced Java, especially if your first Java class did not cover data structures, polymorphism and inheritance. Depending on the content of the advanced class, it might give you interesting software engineering fundamentals, as well as providing you with patterns and solutions that can be readily applied in the real world.
However, as other people mentioned before, Java is very well documented online, and it is very easy to pick up interesting knowledge from books or online sources. Therefore, if you already have a good grasp of more advanced development, you can probably pick up the Java specifics by yourself.
Learning C is a very important part of being a computer scientist and of understanding how things work. Depending on your goals, this might be very important. Personally, I encourage people to take C and asm classes just to learn the fundamental computer science concepts that come with studying those. However, I understand it might not be useful for everyone.
TLDR: Java will give you a good boost on the short term, C will give you the fundamentals to take your programming at the next level.
Perhaps post the course outlines for your first Java class, your advanced Java class and the C class. Mentioning your short term and long term goals might be helpful too.