Not all airbrushes are the same. Some are intended for certain applications, while others are designed for specific paints. Choosing which airbrush best suits your needs is essential.
There are two important questions to ask yourself when choosing an airbrush:
What will I be airbrushing? (T-shirts, autos, fine art, photos, etc.)
Which medium will I be working with? (acrylic, gouache, watercolor, oils, etc.)

Airbrushes are also designed differently. There are single action airbrushes and double action airbrushes. There are gravity feed, side feed, and bottom feed models. Each design has its own unique advantage, and serves its own specific function.

Single Action vs. Double Action

When you depress the trigger of a single action airbrush, both air and paint are delivered simultaneously. The amount of paint can be varied by adjusting the needle/nozzle assembly, but this means you must first stop spraying.

With double action airbrushes, you have greater control over both the air flow and paint supply. Press the trigger down for air, and pull the lever back for paint. This allows you to control air and paint flow independently, thus enhancing the effect without having to pause. Most experts agree that you're better off starting with a double action airbrush, rather than a single action.

Gravity Feed, Side Feed & Bottom Feed

In gravity feed models, the paint feeds into the airbrush from a reservoir or cup mounted above the airbrush. This enables the artist to spray extremely fine lines at a low air pressure, thus allowing for more control. The airbrush is easily cleaned for quick color changes.

Bottom feed models use a siphon-feed system to draw the paint up from a jar or color cup mounted below the airbrush. The jars allow for larger volumes of paint to be sprayed for an extended period of time.

Side-feed models use the same technology as bottom-feed models. However, the paint jar or color cup is mounted on the side of the airbrush. This allows for slower spraying and better vision of the work surface.