The difference between dual action and single action is this: dual action- the trigger controls both the amount of air and the amount of paint coming out. Pushing down on the trigger increases the airflow through the gun, pulling back on the trigger pulls the needle back and increases paint flow through the gun. Single action- the airflow is constant and the trigger only controls paint flow.
I have never used a single action air brush so I can only speak to the use of a dual action, so here goes. I would reccommend buying a dual action to begin with because:
1. If you buy a single action you will probably want a dual action eventually, and since airbrushes are expensive this can save you some cash.
2. If the only airbrush you ever have (like me) is a dual action, you wont know how much harder it is to learn how to use than a single action.
Learning to use a dual action isn't that hard. Just get some large pieces of cardboard and practice for a few hours and you will get the general hang of it. Practice painting general paterns with your airbrush. Thick lines, thin lines, dots that gradually get larger, ect. Also buy some cheap kits and practice your techniques on them. You will pick it up fairly quickly.
I generally like to use acrylic paints in my air brush because of the ease of cleaning, buy you can paint with anything you wish. I have even used fingernail polish (properly thinned) to paint cars with.
I use a Badger 150 model and I am very pleased with it, but it is the only airbrush I have ever owned, so I am unable to compare it to others.