Thank you so much for the reply, I really appreciate your perspective. And no, it's not too late at all. I'm still not sure what to do. On the one hand, LSE is a dream come true and it would be foolish to underestimate its prestige, but at the same time, I just love the idea of studying in DC.
I am from Europe and did my undergraduate degree in Canada. So in my case an SIS education would allow me to get the outsider perspective I guess.
I hope to work for NGOs in Europe or abroad after graduation.
SIS offered me some funding, which makes it the (much) better option financially.
Since you seem to know a lot more about SIS than I do, what is your impression of the school's reputation with potential employers in DC? Is it fairly easy to find internships? If I decide to attend SIS, it would be for the unparalleled opportunities that DC has to offer, given my interests.