I see very little advantage to DC over AC in any but closed loop applications where the ability to vary flow is actually useful. As with all things, the actual answers depend upon your specific implementation, so if you want good guidance it would help to know something about your system and how you'd plan to use the pump. For example, some skimmers need pumps that can develop good pressure, which eliminates all but the most expensive DC pumps. Likewise if you will need a pump to handle higher head pressure. Comparing the energy usage of a high head pressure AC pump to a DC pump is apples and oranges.