Duke is an awesome school but I know someone who went to Duke and did an MD/MPH in 4 years, they did feel a lil burnt out. Durham can be boring for 8 years too .
Dartmouth seems like a great option for you , an ivy experience in an all new place and seems like a great fit for your interests. I would go with Dartmouth as I feel like one should experience new places and lifestyles when they are young . I would also want to see a change in school flavor, but that's me.