Assuming you got full ride + stipend at WWS and little aid at HKS, then I think it is a no brainer in favor of WWS, as WWS is as good as or arguably better than HKS, depending on your preferences.
If HKS is offering significant funding, then it becomes a harder decision. HKS does have an advantage in terms of a greater number of course options, including the option to take courses at Harvard's other proffessional schools. The relatively limited course offerings is really the only shortcoming I can see with WWS. WWS has the edge in other areas, such as smaller class sizes and superior career services (from what I hear), and of course the big one being the ability to graduate with zero debt.