I don't think Cambridge, MA is the main draw for any HKS student, but it is certainly a nice place to be for a few years offering the benefit of an Ivy League campus set on the edge of a major metro area (think Georgetown, not Columbia for the relevant comparison). Although probably not a focus for most two year students, a school-year internship in Cambridge/Boston is definitely easier to come by than in Princeton. I think HKS's biggest drawback (hashed over many times in this thread) is the funding/debt issue. With that as a given, there are advantages to Harvard. For example, if you plan to study development and want a taste of (borderline) heterodox economics, having Rodrik is a big benefit. The econ faculty on the whole is incredible at Harvard (and MIT where you can take cross-listed classes), so if your interests lie in a more quantitative direction in general, HKS might be the better choice. It is not my subject area, but with the med/public health faculty at Harvard that may be another study area where Harvard excels.
Overall, resources are better at WWS. For someone weighing their options between full funding at WWS and no funding at HKS the choice is easy. But if you are fortunate enough to get funded at both schools, then I don't see a big difference in the things that actually matter: faculty, coursework, job opportunities, etc.