The double action is easier to control then the single action, and faster.A double action allows you to control the spray pattern by pulling back on the air control instead of turning the knob at the back of the airbrush. There is nothing wrong with the single action. I have a Badger single action airbrush with the
Badger electric, oil less compressor. While there are times that a double action would have been more convenient, I can't complain about what I have. When you get a compressor, get a quiet, electric oil less one. While a Sears "tank" style might seem like a bargain it will be more trouble then what it is worth. For many years, the two best airbrush's were Badger and Pachymer (I don't remember the proper spelling). Chose one and stick with that brand. Some of the bottles and hoses and equipment won't interchange well. Say away from overseas junk. They will aggravate you at best. Some of the ads in magazines from the 1930's and 1940's are fine examples of airbrushing. Good luck and remember that it takes a lot of practice to get good.