If I might chime in here...all things being equal in terms of "how" a good airbrush should work, there is precious little between the 2 to choose from. I've been "brushin" since 1970 and presently teach it to the members of our local IPMS club, as well as general public who come to my classes at the local art store. Fit of parts, QC of the materials used, and general operation seem to be a touch better than Iwata...IMHO. Both are easy to use and perform all tasks with ease and perfection. Where this gets hazy is when you set up a situation where an experienced user grabs a "so-so" brush and can get a great paint job on a kit, and a "newbie" has a $300 brush and gets a sloppy finish.
As one gentleman put it..."it's just a tool"...!And we all know, or SHOULD know, that a tool is only as good as the know-how of the person holding it. You can't learn airbrushing from scratch overnite. When it comes time to purchase a brush, there are a lot of things folks look at to make their decision.
The MOST often reason for that decision...is cost...nuff said...!
thehermit