Naturally large surfaces get airbrushed. For small parts and areas that might seem to be more suited to a paint brush, I sometimes still use an airbrush, depending upon a variety of factors. If there is a lot of intracate detail that would get lost by a thicker coat of brushed paint, I'll use an airbrush. If the color to be painted doesn't cover well, I may well use an airbrush such as for the yellow leading edges on Spitfire wings or the basic flesh color for tank crew faces.
For an airplane, I would be airbrushing the exterior, the prop, the basic cockpit, the pilot's face, and possibly the engine. The pilot's uniform, cockpit details, gear struts and tires would be brush painted.
For a tank, I would airbrush the tank, it's suspension, wheels, (the tires if its possible to mask), tracks, basic interior, crew faces. Uniforms, details, and pioneering tools would be brush painted.