I thought I'd write about my experiences with my airbrush. I have several; Iwata, Paasche, and Badger brands.
I like them all, and each have their own little quirks. My Paasche seem to leak air at the hose/airbrush sometimes and I have to use the wrench to make it tighter, I can hand tighten my Badger and Iwatas with no air leaks, Badger though recommends a wrench. The skinnier Paasches are precise, lighte and responsive, and some have fatter beefier bodies if you prefer - they are all built well. My V, side feed always seems to clog though. I wish Paasche make their parts hand tighten, instead of using the wrench.
The Iwatas have a good quality build even on the inner parts you don't see, and the nicer chrome finish does seem to prevent paint from sticking. Trigger is robust and needle travel is longer for a more varying line. You can pretty much hand tighten almost all the connectors, which is nice. What I don't like are the higher cost of the airbrush and parts for it. I haven't had to replace any of my parts though, someone mentioned that Iwata parts are more durable, i.e. the needle is stainless steel. But, if you take good care of the other brands - they should last you a long time as well.
If you can only buy 1 airbrush, I think the most versatile would be the Badger 150 (fine and large tips) - you can use the preset screw to make this like a single action as well. Buy the set because it's more cost effective and you get jars, tips, hose, color cup, etc., it comes with 3 tips; smallest is .25, .50, and .85 (or .75) as the largest tip. If you want fine lines, put on the .25 tip, for large spray with thick stuff use the large tip and even move the needle out of the way so you just push down and sprays a lot of material unrestricted. I'm not sure about the 'one tip does all' for a beginner, because you have to be somewhat skilled to do fine lines with a medium to larger diameter tip, unless you have a preset screw on your brush to single action it. True, Badger has that teflon bearing, but I've had my airbrushes since the early 90s and never had to replace the parts - just take good care of them. The Vega 2000 is also good, but a bit larger nozzles. Tip sizes for the Vega 2000 is .40, .75, 1.00. It is like the Badger 150, but with a floating nozzle instead of screw in, also no teflon bearing to replace.
You can make almost any double action airbrush a single action, even those with no preset screw - just unscrew the needle in the back and move the needle back til you get your desired spray, then tighten.
If want 2 airbrush, for the most versatility, get 1 gravity feed (fine tip) and 1 siphon feed that can take the large tips. You can get the Badger 150 and 100 series (they can interchange parts, tips, and hoses). Or a Vega 2000 (or Badger 150) with an Iwata gravity feed with fine tip. Or Paasche H (coarse and broad spray) with a gravity feed fine tip airbrush. Whichever you get, you should get 1 airbrush for fine and 1 for broad spraying. Gravity feed is nice for fine since there's no bottle or color cup hanging down getting in the way.
You can use the larger tips on a Badger, Thayer, or Paasche, to paint/stain/varnish lawn furniture, picture frames, window and door trims, tables, chairs, refinish the kitchen trim, etc. and not just restrict it to modeling/illustration stuff. They do an excellent job and the clean up is easier than using hand brush or roller.
I believe you can do really nice work with any of the airbrush mentioned above, you have to learn your tool. And when all else is nearly equal, cost is a big deciding factor for me that is why I prefer Paasche, Badger, Thayer Chandler.