Squared off tips are less efficient, but allow higher solidity and therefore more power can be delivered to the airflow by a propeller of given diameter.
C130 - short field takeoff is a primary performance requirement, so high power output is necessary.
P-3 - oceanic endurance is a primary performance requirement, so efficiency becomes the overriding concern.
All the above is just off the top of my head, so I await correction.
O8