The main benefit of rocking piston pumps is for ponds greater than 18ft deep. At 18ft with a rotary vane get the diffuser off the bottom on some sort of stand 12"-24". Both (1/4-1/3hp) dual head rocking piston and rotary vane produce similar amounts of cfm. Rotary vane is easier and simpler to rebuild. Rocking piston is often slightly more retail cost.

aka Pond Doctor & Dr. Perca Read Pond Boss Magazine -
America's Journal of Pond Management