I have used the Aztek before, and really acheived some good results with it.
The problem with the Azteks is they tend to break internally, causing paint to leak inside the body and gum everything up.
They also CANNOT spray lacquer based paints, the lacquer eats away the rubber 'O' rings, and from there it's pretty much stuffed.
It is a bit of a mongrel to clean also, cause you cant strip it down to the bones like the Iwata and Badger.
IMO the best bang for buck of the 3 is the Badger, giving you an airbrush that with some practice will deliver fine lines and decent atomization.
Of course you will have to purchase tips to suit the work your going to do, fine, medium etc.
I have only read about the Iwata, and they are a good quality airbrush, but they are also pricey to buy, so that bit is up to you.
Personally I am after a detailing brush, and at this point in time, it looks like the Badger 100G is going to be my choice, with the Fine needle ass'y.
I currently own an Omni 4000, and it is great for general use, but the large fluid cup, and the general needle ass'y makes it difficult for me to attain certain paint finishes like squiggle camo etc.