I had a similar dillema before starting my own game for my final year university project. I recommend you actually try both to begin with. It won't look good on your report if you have to say "We decided to go with Java because we asked some guys on a Java gaming forum what the best language was and they said Java was way better than C++". It looks much better if you actually go through the process of weighing up each language's advantages and disadvantages yourself. Then, depending on the type of game you intend to make, devise a test application that you can write in both languages that tests each of the language's limitations. If it's performance that you're worried about then it should be pretty easy to test. If it's speed of development then you can measure that too.
It helps as you go through your project if you plan each section of your report as you develop the project. At each stage think about how you're going to answer the "We did this in this way because..." questions.