you can make a strong argument for either side. missing on your list of ratting him out: in some ways, it is the “right” thing to do. you are helping the justice system work, and it works on this very kind of situation. people get immunity all the time for cooperating with police to allow them to get a conviction on someone else. i know your situation is a hypothetical, but could you modify it at all? because as it is, its unlikely that if the two of you are partners, and neither is the mastermind/a bigger criminal, that you would be offered a deal. is it a set situation? if not, i would say that for some reason, they want your partner more than you, or maybe they cant prove much on you, but they can on him. etc. maybe this doesnt impact the argument at all, i dont know how your debate works, but the other side could fault your situation, which would surely work against you.
