Airbrush: Applies an overall surface coloring, good for fading, and dust/dirt buildup. Airbrushing "coats".
Wash: Applies a thinned color that will tend to fill "low" areas, like the space between boards, or corners like between edges of braces or trim and siding. Washes "fill".
Drybrushing: Applies almost dry paint by "scrubbing" the color off the brush. This accentuates raised areas, like rivets, edges of angle braces, edges of corners, etc..Drybrushing "highlights" the small details.
Actually, a combination of all three methods will provide the greatest control of the overall effect of weathering. The most logical order would be wash first, to fill in. Drybrush, to accentuate. Airbrush, to coat with dust/dirt, and to lastly, dull the finish. When drybrushing over decals, take care not to accentuate the edges of the decal film. This isn't as important when drybrushing "body color" over the decal, to "fade" it, as it is when adding dirt streaks.
In the end, ask what weathering am I applying? Is it dirt deposits from rain? (wash) Is it wear damage from scrapes or wind blown abrasives? (drybrush) Is it "color" that sticks to the structure, like windborn dust, or pollen? (airbrush) or , is it sun damage, fading? (airbrush)
Regardless of the procedure, you will have to practice. When you see the "toy trains" on sale?, think "practice project for one, or more, of the weathering techniques"