Why is an airbrush better than a spray can?
Well, it's not necessarily better, just different...
Spray Cans:
>Pros: Easy, easy, easy and easy.
>Cons: Can't mix your own colors, can't adjust spray pattern.
Airbrush:
>Pros: Can use any color you can find, can adjust air and paint flow,
>Cons: Must clean well after each use, dependent on a propellant*, may have to re-fill for big jobs
*Propellants: Compressor or compressed air can.
Compressors are expensive: $100+, but you don't run out. It's a one-time investment.
Propellant cans are cheap ($7-10), but only last for about 3-6 models worth of spraying.
I most often use a spray can for the car's body color, and airbrush all the other details. Since I use Tamiya paints, an advantage to this is the two won't mix. Tamiya sprays are synthetic lacquers, and I use the acrylics through the airbrush. That means that once the base lacquer coat is down, if I mess up on detail work with the acrylics, I can get them back off without hurting the lacquer. I use either the acrylic thinner or window cleaner (with ammonia).