Existentialism is just one branch of nihilistic philosophy, one which specifically attempts to address the issues inherent in nihilism.
I think it is more accurate to describe existentialism as a reaction to nihilism, not a branch of nihilism. Camus opposed nihilism. It is true that he (and other existentialists) took nihilism very seriously indeed.
Happy New Year! Here's the latest and greatest installment of rationality quotes. Remember: