My personal vote would be for Duke.
From a professional standpoint:
In your post, you seem to focus on one individual at Cambridge who you would be working with, while you mention there are a host of individuals at Duke that you could potentially collaborate with. I realize this may be less common in the UK, but what were to happen if this individual left (for, say, somewhere in the US? Or to the continent? Or took a sabbatical)? You can always carve out your own niche, but having other people working on similar issues that you're interested in will make it considerably easier to carry out your research. What if, after working for a few months, you find that there's a personality clash between you and that professor? Would you be able to switch supervisors?
From a personal standpoint:
If you're worried about intellectual culture: Duke, like any other major research university, cares about nothing but the research you put out. You may find a few supportive peers and mentors - but the focus is always going to be on the work you can produce.
Life in Duke and Durham is going to be very different; the South has a very distinct culture and the 'Triangle' research area has its own culture on top of that.
From "further down the road":
Having an international scholarship certainly can't hurt career prospects down the road. What are the funding trends in your field? If you stay at Cambridge, are you going to network with many of the same lecturers and professors that you were exposed to as an undergraduate, or will you have an equally good opportunity to network with an international group of researchers and graduate students as you would at Duke?