There's really six things to choose from (other than price):
Single vs Double action
Siphon or bottom feed vs gravity feed
Internal mix vs External mix.
MusicCity has an excellent article on
Airbrushes. He leads you through the various features of the above.
Some people argue that single action are easier to use than double action. Personally I don't see it. I think if you can use a single, then the double is not that much harder and the benefits outweigh the difficulty in controlling the paint flow. I think of them as single pole light switch (light is off/light is on) to a dimmer switch (light is dark, less dark, more light, full light). The double action can do everything a single action does, but the single can never be a double action.
There is no "best" though each have their fans. I was given my first AB (an ancient Badger 150) and bought a new 150 so I have interchangable parts. The fact that more than 20 years seperates my brushes in age, but not in features is a testament to the design. I also have a preference for US built products (even though I'm Canadian!). Iwata fans are almost rabid in their praise, and I'm sure that the brushes are every bit as good. They do cost more to purchase and buy replacement parts, and that cuts into the kit buying, doesn't it?
People will argue that gravity fed brushes are "better" because they use less pressure, and I can see that. But I also think they can have their issues with balance while using them. I will eventually buy a gravity feed brush (I can't make up my mind between the Badger 100G or 100LG) to augment my 150, but it isn't vital that I do.
Really it's up to personal preferences. If you can find a hobby or art store that stocks airbrushes see if they will let you handle them. They probably won't let you squirt paint through them, but you can get an idea of weight, balance and smoothness of the actions.
Read the rest of
MusicCity's Website and admire his models. He is one of the masters around here. He'll probably be along shortly denying that, but don't listen to him!