Nick - what type of modeling do you mostly do? Both will put out mighty fine lines which I imagine wouldn't be easily distinguished just by looking at it with the naked eye or without a grid or incremtns to compare to... If you do a lot of 1/48 aircraft that type of detail might be a little overkill... nice to have but not absolutely necessary. Either one would work fine for 1/72 I imagine as well...
Basically it's not the width of the line but the ability of the airbrush to keep the smallest area of paint delivery with as little overspray as possible that is important to us in the modeling field as far as detail work...