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You'll want to choose loans that have the ability to delay payments for your residency (so those targetted at med students), and you'll probably want to pick the company that has the most options for hardship deferments in case you need them.
Personally, I would choose a fixed interest rate. Rates are very low now and basically can only go up in the future. In your situation, you are going to have this debt for many years, so the fixed rate is probably the better bet.