Diffusers create tiny bubbles that, as Steve mentioned, have MUCH more surface area than bubbles typically created by reactors. These 100% CO2 bubbles will immediately begin diffusing CO2 because of the lower CO2 content in the water column. Likewise, N and O2 will diffuse into the bubbles because of their lack in the bubble. When the bubble reaches the water surface, it will be mostly, if not completely, N, O2 and other gases. The CO2 will have diffused into the water.
In my experience, I have not found power reactors to be more efficient than a good diffuser. Yes, they trap the bubbles until all CO2 is diffused, but is that really necessary? Are we so sure that a tiny CO2 bubble created by the diffuser doesn't release 100% of its CO2 also? Perhaps even quicker?
Power Reactors are bulky, unsightly and require a powerhead to work. The addition of another electrical gadget to me is not worth it.
Diffusers can be well hidden in the aquarium and are easy to clean. I simply dip it for a few minutes in a water/bleach solution for a few minutes every month. Not much to it.