keilau
Bill suggested that all airbrush are the same in a good user's hand and all can draw very fine lines, therefore the nozzle size and spec does not matter. I disagreed and said that the ability to draw fine line is not necessarily the sign of a "good" airbrush.
The balance in hand, the feel of the trigger control, resistance to clogging and needle damage are important to the selection of an airbrush. These are all subjective, but they can be traced to the construction and manufacturing quality of the airbrush. The history and working principles of an airbrush is well know, not a black art.
In a modern internal mix airbrush, the air and paint are mixed inside the body of the AB and then flow throght the head between the nozzle and needle to achieve fine atomization. A smaller nozzle and longer needle taper achieve finer results. Clogging begins at the needle tip with a small amount of dried paint, then grow and eventually clog the airbrush. A polished needle is less prone to clogging. Some interesting pictures by Zsolt demonstrate that. He also proposed a solution. While these measures by the professional are extreme, but the effect does give us some insight into the working of an airbrush and is NOT irrelevant to the modelers.
One airbrush manufacturer makes their nozzle holes large and the needle taper short to improve resistance to clogging and needle damage. Another uses harder material and polish the needle to achieve the same. The later is able to use small nozzle hole and longer needle taper. Do these two airbrush work the same? No, they don't. Do I have a preference between the two? Sure I do.
I know that an Iwata Eclipse HP-CS does not work the same as a Badger Patriot 105 because I have both. Both should be good enough for 99% of the modelers, but they ARE very different due to the 2 times larger nozzle of the Patriot 105 fine needle and the 2 times longer taper of the Iwata Eclipse.
I don't know if the Badger Velocity will work the same as a Harder & Steenbeck Evolution because I do not have an Velocity, and acquire an Evolution only recently. They have similar nozzle/needle size of 0.2 mm plus/minor 0.01.
I know that Bill will say that "Kei, you are taking this too personal." 
Kei, you are taking this, oh never mind! 
Please, I have never said that "all airbrush" are the same, only that we shouldn't get caught up in specifications and presume that a small nozzle size is a sign of a good airbrush, or is more capable of spraying a fine line. As you said, for 99% of the modelers any airbrush is good enough!
Please go and re-read the article I linked to, three of the airbrushes are Iwata: BCS, HP-CS and a Micron B, with the fourth being a SATA (completely unknown to me, but fairly pricy). Not a cheap knock off in the bunch. The thrust of the article was that a $90 BCS or a $120 HP-CS are equal to a $375 Micron B. You can buy a compressor, a kit and all the paints and the supplies for the difference.
What does matter is practice, practice, practice!
I've never recommended the cheap knockoff airbrushes, and never will. But I do, and will continue to, recommend buying a $75 airbrush over a $120, as many modelers are on lean budgets. However, in the interest of fairness I often point out the range. I lean towards Badger (and with some judicious shopping you can get one of their excellent airbrushes for under $50), but have also offered up Iwata and Paasche as alternatives.
Harder steel makes zero difference to an airbrush's operation. The Iwata uses a "harder" steel because they emphasize a narrow diameter needle, which makes them far more likely to get damaged during manufacturing and handling. Small nozzle size can affect overspray, which can assist in fine line spraying, but that has as much, or more, to do with air pressure. Longer taper allows for slower transition between less and more, which can also assist in fine line, but neither dictate the fineness of the line. Polishing a needle reduces clogging, which can also cut back on over spray, but a polished needle isn't any more capable of a fine line than an unpolished one. It is a balance between paint viscosity & thinness, air pressure and air flow that makes the biggest differences. But far more important is the user practicing their craft.
Is there a difference between a HP-CS and a Patriot? Sure there is. According to Dixie Art around $45! The longer taper of the HP-CS makes it easy to control the line width transition compared to the "new head" double taper of the Patriot. I'm not a fan of the design and prefer the older 100 series head (I've only used a 175 for comparison, the transition between no paint and WHOA PAINT is short!). The Omni and Vega have a longer taper still, but I prefer the trigger action on the 100LG. But those are personal preferences, and make little difference in performance.
However, it can make a huge difference in use. Performance and use not being the same thing. If you are happier using the HP-CS, then you are far more likely to use it, and therefore will become more proficient through practice. And that is what is truly important. An airbrush, no matter how expensive, or well built, or finely engineered, is completely devoid of lifting itself out of it's box, and spraying a line. It takes a user to do that. And I've seen people use the "lowly" Paasche H that easily put my best work to shame.