| Passport is good if you eat every meal on campus and don't eat a la carte. Basically, Passport pays for a meal in a dining hall or the equivalent amount of food at an a la carte place. However, you can only eat between certain times, and if you eat a la carte, then you can only spend a certain amount of money (you can pay for extra food through a snack account).
Diplomat is a declining-balance account that can be used at any time and at any location. It's especially good for buying food from markets.
I would recommend Diplomat for basically everyone. It's much more convenient, especially for those with schedules that preclude eating during "normal" meal hours. I've said this several times before, but all of my friends have Diplomat, and I don't know anyone who isn't a freshman who still has Passport (I'm sure they exist, though). | 
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