Amherst, along with Vassar, has the greatest socioeconomic and ethnic diversity of any top LAC. Although it has a preppy history, those days are long gone. The typical class will be smaller than the same class would be at Tufts, and the access to summer internships and unique career options will be greater. The alumni network is small but incredibly supportive.
Tufts also is an excellent school, and if you require a more urban environment, it fits the bill. There is more research going on at Tufts, but there are more students competing to do that research, including grad students.