they are both secure, to a degree.
roll up doors, can be VERY secure, if you back up to another trailer, or a wall. just not necessarily a dock.
swing doors are VERY secure if you back into a dock, or against another trailer or a wall.
at my company, we use roll up doors, i like them over swing especially in bad weather.
but if the cable breaks, good luck in lifting that door....~!!!!~