As long as you schedule an appointment, it (in theory) makes no difference whatsoever where you do the interview. Since you're flying in December and have plenty of time, try to schedule the interview at least two months ahead of time. That way, you will have time in the possible, but unlikely event that CBP makes a mistake in processing your membership. Otherwise, do whatever is convenient. And remember that wherever you choose initially, you can easily change interview sites later, as long as there are open appointments.