Thanks, Journeyman, and thanks, Rainman, for your two what I believe to be complementary ideas from your observations. To me the two bubble sizes seem to be two useful tools, and neither mutually exclusive.

In moderate conditions such as journeyman has experienced this winter (only "moderate" compared to what wbuffet's pond experiences nearly every winter, of course), the greater volume of water moved by the membrane diffuser keeps as larger hole open.

On the other hand, the Colorado mountains get a few feet of snow in one storm, filling the open hole with snow when the solar aerator and maybe the wind driven pump are not working. The sub zero temps that follow the storm then set up that snow into ice. At this point it may help to have the more aggressive action created by the proportionately larger bubbles to cut through that ice (and snow) cap to reopen a hole.

What?! Another "it depends"?! Just a couple of thoughts, and possibly not worth 2 cents.