While I don't know enough about Iwata AB's to recommend any (still using a budget Tamiya basic AB which has served me pretty well so far), I do recommend getting one or two single action AB's, and one double-action AB, if you do plan on buying more than one.
The reason behind this is that single-action ones are simpler and have less parts to clean. They're useful and convenient for simple one-color paint jobs or priming. You can buy a couple of those and load both up with different colors. That way, you don't have to clean out the AB to spray another color in the same session.
And since you'll be buying more than one, it's good to stick with single action for this purpose because they're cheaper than double action ones, and you don't need to do anything fancy with straight-up paint jobs.
The double-action AB is best used for effects such as post/pre-shading, filtering or painting exhaust stains, due to the ability to control the air pressure and amount of paint. As you probably already know, double action AB's require more skill to use and require more time to clean due to the extra parts.
Some people complain about getting finger cramps when using traditional "pen-type" AB's. I don't know how true this is, since I use a trigger-type AB. Both Tamiya and Iwata (who makes the AB's for Tamiya anyway) have trigger-type AB, so you might want ot look into those, too.
They'll be somewhat more expensive, though.
Hope this helps some, even if I wasn't able to answer your main question.
