The faculty at Cambridge is great. But I still recommend Tufts. Cam Mphil is too short. You have to apply for PhD programs at the very beggining of the first semester. So you won't have enough time to polish your writing sample and get familiar with professors (even your own supervisor) so that they can write strong LORs for you . Therefore, Cam Mphil won't be very helpful to your PhD application. But if you've already decided to apply for PhD after you finish the one year mphil course, then I would say Cam is a great choice for you. Many cam mphil students end up doing this.
I also doubt that Cambridge has a higher presitge. Tufts is Tufts.