That's definitely a tough one. I'm definitely more biased towards Cornell, but Duke is still an excellent program as well.
I'd like to say that Cornell's programs (such as the Parker Center, Cayuga Fund, and the Stock Pitch Challenge) help Cornell stand out above Duke, but honestly I don't know too much about what Duke's offerings are in the way of finance.
I think maybe a great way to learn more about IB prospects coming out of these programs would be to reach out to alumni from both of these programs that work at a BB. A lot of times listening to their personal experiences with recruiting can help you get a better idea of what to expect from each program.
For example, I spoke with a friend of mine who was a Tepper grad and works in IB now. He told me how going into Tepper he thought the opportunities would be readily available and that recruiting would be easier than it was. When he was there, the network was not that great and he had to do a lot of the legwork himself without the assistance of career services or alums. Fortunately for him he was successful and he's helping to expand Tepper's finance network, however that's definitely something that Tepper doesn't advertise on any of their brochures.