My daughter just went through a similar process of deciding between Dartmouth and Northwestern. Northwestern gave her a better offer in financial aid (not as great as full ride as you received from Duke) but we eventually chose Dartmouth because she loved the peer admitted students better after visiting the admitted students day.
By saying so, I may still advise your picking Duke in your case. The financial aid difference is simply too great to ignore. We love Dartmouth very much and wish everybody loves it. But there is no question that a Duke full-ride oppotunity shall win. One of my daughter’s friends rejected Harvard by taking the Duke full-ride last year. Hence I feel this should not be a question at all.
Good luck!