I have been accepted to those three universities. I am a lawyer from Colombia. I currently work in a small boutique firm that mostly works with Colombian clients, but we also work with a couple of international clients (mostly Chinese or European). The bulk of my work is mostly corporate law and compliance. My long-term goal is to stay in the US on a sponsored visa. I know that with only the LLM, this is unlikely. Therefore the possibility of transferring is to their JD program is an important factor.

Gtown.
In Colombia, Gtown has the best reputation, employers put it on par with Harvard.
If I were to return home, this would give me the biggest edge. However, I don’t have a fellowship or a scholarship, and I cannot transfer to the JD.

NU.
Northwestern is well-known in Colombia not for their law prowess, but for its management school (Kellog). But the LLM/Kellog program has been suspended. On the other hand, the university has granted me a 40% fellowship. NU won’t allow the transfer either.

Duke.
Of the three Duke is the least well-known. They have given me a 40% scholarship and would allow me to transfer to the JD.

I find the Duke and NU program very similar corporate-wise, both have great professors are highly ranked (11 and 12), but in terms of International Law Gtown is way above them.

Given what I’ve said, I would appreciate if you guys could give me an inside scoop and any advice on how to make this decision.

I think you should go for the one that offered you a scholarship, since it is a lot of money and all three are very well known universities. Maybe think about which city you would like to live the most?
However, if you want to do a JD, then you should think about re-applying this year, to start a JD next year. I think most of us won't do a JD since it's more expensive and it would be least 2 years without working, without an income, whereas LLM is only one year and cheaper. You would have to do the LSAT, they offer it outside of the US on very few dates, I think you can check it here: http://www.lsac.org/jd/lsat/test-dates-deadlines
In the end, no one can really help you make the decision, you will have to take risks and decide it yourself...