Hi Ishi,
Both the places have their pros and cons. I am not sure about IO's in Geneva but from my work experience with UN, you will have an advantage with a Dev Studies degree over International History. It really depends on what you want to do afterwards. Oxford brand name will definitely carry weight. IHEID has an excellent reputation in Europe but it's not as well know internationally. If you're looking to work in International Development and money is not a big concern, go with Oxford. My two cents. Do talk to as many current students as you can and get more insight. All the best!