Lets say you have to pick between the two. You [i]will[/i] get attacked one way or the other but which would you prefer? You can't just be like "Nuh im 2 cool 4 sherks or allgatres"
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Shark, since usually it's just the one bite, then they leave you alone because they thought you were a seal. Alligator seems like it would be after the kill, plus I'd imagine it would hurt more to have it tearing your flesh rather than cutting.