A jellyfish has nematocysts on the tentacles that allow it to "harpoon" prey items that contact it. To assist in moving prey to the tentacles, jellyfish swim using their muscles and nerve net to coordinate movement. This movement has the effect of pulling food particles toward the jellyfish tentacles. A flatworm on the other hand, has chemosensors that detect prey items, a brain to coordinate input from sensors and muscles and cilia to propel the flatworm toward prey. Once it has found the prey, the pharynx will move food into the gastrovascular cavity.
The flatworm would be the better hunter because it not only has effective tools to handle prey, but unlike the jellyfish, it can move toward prey. The jellyfish has to wait in hopes of encountering food items.