I've never used an Iwata but have heard great things about them.
I have been using the Aztec for the last 7 years or so. I know a lot of die hards out there dismiss this airbrush as a novice style piece of equipment, but I have used the Aztec in painting $50.000 00 Harleys, Goalie masks, refrigerators, displays, mailboxes, vehicle graphics, etc.
I just recently did a flame job on a 69 1/2 Camaro (plastic model) and it turned out great. The ease of use with the Aztec makes it my choice. Cleans up well, and is darned versatile. All of the nozzle sizes are there and cleanup is a breeze. (Just make sure to soak the body after a heavy round of spraying). This gun will handle just about anything in terms of paint and works well with any air source.
For this Camaro, I used the Metal Craft line of paints for spraying the basecoat and then applying the flames...all in one day, using 3M vinyl as a mask. A simple hair dryer was used to quickly heat-set the paint and there was no lifting or bleed through whatsoever. I was quite surprised, considering I have not done a lot of models in this fashion.
Anyway, just thought I'd pass along my thoughts on the topic. New to the site and it's very nice...lots of info!
Thanks
Kim