@Kaleb - Basically, different needle sizes needs different tips/nozzles. The larger needle/tip set-up, the more paint will spray through the AB. Say, if you want to paint free-hand camo on an airplane or tank with very fine details you would probably do better with a 0.25-0.30 mm set-up. But, if you´re about to paint a car model body I would instead choose an AB with a 0.5-0.7 mm set-up.
Still, you can paint some very fine lines with the larger set-up but it´s a bit easier with the smaller set-up.
Like Erik Smith says - a single action AB is quite sufficient. Especially if you´re painting larger bits like a on a car model. The Paasche H or the Badger 200 NH, for instance.
Like I wrote before, I like AB´s which has drop-in tips/nozzles (Iwata Eclipse, Badger, Paasche etc). The other type is the screw-in tip/nozzle which is much more fiddly to install and, sometimes, easy to damage. Doesn´t mean it´s a bad design - just different and in my opinion the drop-in tips makes life a little bit easier.