For open water trolling the P66 offers a wider cone angle and dual frequencies. The wider cone angle allows you to see more under your boat. The GT56 has a high wide CHIRP frequency. This gets you a 16-24 degree cone angle. The P66 on 50khz can get you 40-45 degrees. It's almost double the size to allow you to see more.
These fish finders with pre-packaged transducers are good all around transducers. but they are geared towards inland fisherman who bass, panfish, and walleye fish shallow water looking for structure and bottom detail. For trolling guys are in a large lake in open water looking for pelagic fish, so the bigger cones allow you to see more.