Hi there Brian
the basics.. I'm sure more people can add to this.
most single action airbrushs are controlled by a on/off trigger.. press and you get paint.. let go and its off..
A double action gives you a little more control in the amount of paint you spray.. pressing down turns on the air but pulling the trigger back engages the paint.. the further back you pull the more paint you get out.
I started out using single actions but gave them up when I got my first double action badger.. the learning curve is there but not that much more difficult than a single action.. the hardest part for me was learning how to thin my paints..
as for recommendations.. everyone has their favorites.. I've used Testors and Badgers and my favorite and basicly only airbrush I use anymore is My Badger Universal 360.. (same as a badger 155 except it can work as both a siphon and gravity feed airbrush..)
I like the badger 155 / 360 airbrushes cause there is only one tip and needle size to worry about and its very easy to clean after use.. (average 5 mins for me to clean up..)
The badger cresendo 175 is also a good brush (which i also have) but is a little heavy duty for modeling its pretty good at putting on basecoats or airbrushing future.. but I don't use it for fine work..
just my 2 cents.. I'm sure everyone will be jumping on the bandwagon and I look forward to reading more on this thread.