When choosing a compressor, the most important thing to determine is what you'll be using it for. What are you airbrushing? T-Shirts? Automobiles? Fine Art? Once you've established that, it's imperative to also make sure your compressor will be compatible with your airbrush and the types of paint you use.
The air compressor is your gas tank; it provides you with the reliable, steady air supply you need when airbrushing.