Williams College or Johns Hopkins University for Poli Sci & Econ double major?

OP: Both are great options. Really comes down to your personal preference regarding environment.

Do you have any career objective ?

Do you enjoy research ?

Although there are only about 40 masters degree candidates in residence at JHU for econometrics & quantitative application, it does make a difference. (Most JHUecon masters degree candidates (several hundred) do so through distance learning.) Whether that difference is a positive or a negative depends primarily on your preferences.