If everything's the same to you (cost, size, flexibility, etc.), then I think the clear answer is BU. Northeastern's program is only for urban health and their classes focus on the community side of it, not too much in epi. Tufts has many tracks, but their epidemiology and biostatistics ones are combined. I think BU will allow you to specialize what you want study a lot more since there are just so many options for certificates and tracks. I'm assuming you're already in Boston... you could knock out a visit to all 3 in one well-planned day and see for yourself.