Hi there, I'm a current GW policy student! Being in DC for health policy is really just the place to be, in my opinion, but I admit I am biased. However, the difference between 60 and 77k is a lot of money, and I can understand why you're torn.
As someone who has lived in both Boston and now DC, I like both cities but prefer DC. The difference in ranking doesn't mean as much as the location - e.g. it's easier to get a job in DC if you go to school there, and same with BU. Where do you want to end up?
Also, I'm not sure how BU works, but the vast majority of GW classes start after 4pm, so you can work full or part time which can significantly ease the financial burden. I work full time (was working full time before starting school) and I go to school part time. Plenty of GW students go part time if they work a lot.