A week and a half is honestly too long for either city. If you can rent a car then you do have fantastic day trip opportunities in both. You could also do a train tour and feasibly do Boston, NYC, Philadelphia, and DC all in one trip…. Also since haven’t decided, have you considered the West Coast? I think it’s a much more interesting place to spend a week and a half. Look into San Francisco, you have tons of history, gorgeous nature, excellent food and great transit, and there’s a ton to see and do in a week and a half.