I think most aerators are pretty efficient, but of course there always seem to be a "green" model of everything out there nowadays.
The size of diffusor will determine the size of aerator. The simple little aquarium aerators sold in the pet stores are used quite a bit, but it all depends on the volume of diffusor you are wanting to use. If the aerator is something vital for your system, then look into getting an aerator that have multiple diaphragms. Diaphragm is the aerator part that experiences all of the wear and tear and, depending on how ya use the aerator, ya will likely experience replacing these once a year. A multiple diaphragm aerator allows you to use the secondary diaphgram while you're waiting for replacement parts for the primary diaphragm. Air lifts are often used in filtration systems instead of pumps so a multiple diaphragm aerator becomes extremely useful. So, whether choose a multiple diaphragm areator or not, it will depend on how vital the aerator is to your pond system.
What are you expecting the aerator to accomplish for ya?