I think most modelers would say to go with the Double Action... it's a little more to get used to, but has a lot of advantages.
BUT... I have and use both (as do many I suspect) I use a Badger 200 single action for primer coats, future, and finishing coats. When laying down a general coat, it's easier to just set it and go. It is also good for putting on a lot of similar details... like if I need a lot of small dots all about the same size.
I use a Badger 360 Universal for most everything else. The variability is great, and I can adjust as I go.
I had an Aztek, and liked it a lot... but it was a bit more effort to clean it, and changing heads all the time was a bit much.
Another MAIN thing I would say to think about tho, is getting a compressor WITH a regulator! Being able to control the air pressure is essential!!!!
www.naplak.com/modeling ... a free site for modelers
www.scalehobby.com/forum/index.php ... a nice Modeling Forum