Bob - both airbrushes are good... I have the 155 myself and love it... from wide coverage down to tight detail it does it all.. I plan on buying an Iwata HP-CS soon as I want something that does a little tighter detail, for example some of the squiggle German camo....
Basically either would work... there are some that say there is no beginners airbrush... start out with what you think will do the job for you and learn to use it from the first... you will only get better with practice...
The other part of the crowd says start out with a simpler single action learn to control the movement of the brush and the basics of how to spray then move into a double action and learn to control the paint flow.
I am one of the first group, I started many yrs ago with a Badger 350 single action external mix, then came a 15 yr hiatus. I tried a 350 again and didn't think it performed as good as I wanted so immediately (same week) got a Badger 175, and 2 weeks later a 155... So basically I came back to modelling on the 155... every time I spray I learn something new about controlling it or some little trick that makes me better... personally I have not been ashamed of any painting I've done with it from the first time I broke it out...
What Rick said is true too... the single action is easier to use and control, not sure how much easier it is to clean as I have been with my 155 for some time now and it is a 3 or 4 min cleanup... but either will do you good.
I think you'll just have to decide which sounds more appealing to you.,.. learning on a single action, then after you reach a level of confidence graduating to a double action... or starting on a double action and accepting some sort of a learning curve until you get it down...
either way you have all of us to come back to with questions and we're more than happy to help out!!! There's a
lot of talent in this GB alone... I am sure either way you will learn at an accelerated pace compared to someone on their own trying to learn airbrushing!!!