When selecting an airbrush, you should consider several factors: single vs. dual action, top or gravity fed vs. siphon fed paint cups. Also what type of paint you'll be spray thru the nozzle and how fine a line or spray pattern you want.

Single Action Airbrushes have an on/off push button control.

1. Good for spraying a solid fill area, with no blending, such as tanning sprays, tattoo paints, hobby models, some cake decorating.
2. You can control the width of the spray by adjusting the air pressure and how far or close you get to your project.

Dual Action Airbrushes have a push and pull trigger control.

1. Allows greater control for blending and fading. By barely pulling the trigger you can create thin lines, up close to your project. By pulling the trigger all the way you can spray a wide area, holding the airbrush further away from your project.
2. Most airbrush artists & craftsmen use this type of airbrush, because it's so versatile. 
3. Any project will be well served by a dual action airbrush, from small detailing to wide areas of paint.

Top Fed or Gravity Fed Airbrushes

1. Many automotive airbrush artists use the gravity fed airbrush because it works well with metallic paints. Metallics will settle in the bottom of the paint cup and will not completely dispense in a siphon fed cup.
2. Some drawbacks are that you will have to reload the paint cup more often if you are spraying a large area.
3. Another drawback  is that depending on how close you get to your work you may have your view obstructed by the paint cup.
4. Most airbrush artists like the dual action top fed models for fine details because they usually have more control over line widths and thicker paint viscosities.
5. The top fed airbrush requires less air pressure to spray the paint than a siphon fed airbrush; reducing some amount of overspray that is usually created with higher psi air pressure. 
6. Most automotive airbrush artists use this in conjuction with other larger spray guns.
7. The favorite top fed airbrush among most airbrush airtists is the Iwata brand.

Siphon Fed Airbrushes

1. This is the most popular airbrush, because you can interchange several paint cup sizes depending on the project.
2. You do not have to reload your airbrush as often.
3. You can set up different color paint containers, for quick changes in the middle of your project, without stopping to clean your paint cup first.
4. Most airbrush artists use the dual action siphon fed due to its versatility: fine detail to wide areas with quick changing of colors midstream.
5. The favorite siphon fed airbrush among most airbrush airtists is the Paasche VL  brand. (There are generic versions out there.)