There's only one thing about Dave's post, and it's probably just a matter of semantics. I would say that a bottom diffuser doesn't neccesarily provide oxygen faster than a circulator if you're thinking about initial effect right after setup. If I remember some of Cary, Ted and Sue's great posts correctly there is a lag period after setup of the bottom diffuser where oxygen temporarily decreases as the anoxic bottom water and all of the bad gasses get lifted into the water column.
I'm also paraphrasing the experts here, but it is my understanding that you get a lot more water movement per dollar spent with a bottom diffuser because you are using the lightness of air to lift the water. Once you've paid for the energy to drive the air below the surface of the water, physics takes effect and air has to go up. That is essentially free energy.
On the other hand, if you set up a circulator at the edge of a pond, and it's properly positioned, it can spin the water around the pond and effectively use the momentum of the water coming from behind the circulator.
My uneducated instinct says that the cost per unit of water moved each unit of distance is significantly cheaper with a bottom diffuser.
