I like the multiple benefits of a PH diffuser. You get a current underwater to help prevent dead spots, the small size of the PH, and a really good diffusion method that also gives a pseudo bubble wall (though much smaller and more dispersed). I run an air stone on the end of my tubing that runs into the PH. This creates small bubbles so there is no noise from the bubbles getting chopped up (less wear and tear on the impeller as well). I have a solenoid on the regulator so the PH, lights, and CO2 all turn on and off at the same time (9hours per day lighted).
The key thing to proper CO2 levels is surface area (smaller the bubbles the better), and contact time (longer time in the water before the bubble reaches the surface). A PH setup like described above gives you very good of both (extremely small bubbles due to the air stone prior to going into the PH), and a long contact time with the water if the PH is placed low in the tank (mine is about 2" above the substrate).
The only issue with the PH dispersion method is that it needs to be cleaned periodically as it will suck up debris since its near the substrate. The reactor probably wouldn't run into this issue as much since its normally higher in the water column.
Simple, cheap, and very effective.
