jo-
You're on the right track - don't feel overwhelmed by the world of airbrushes.
If you already have an air compressor, you're one step ahead already. (However, your air compressor should be one that has an air tank, and not one of those cheap plastic ones that you plug into a car cigarette lighter). The point of the air tank is so the motor isn't running constantly and to provide a more stabile constant air pressure. For airbrushing, you will need to make sure your compressor has a pressure regulator (so you can adjust the air pressure to the airbrush) and a moisture trap (water in the air line can ruin an airbrushing paint job). If your compressor doesn't already have these parts on it, you can get them from Home Depot, Lowe's, Sears, etc.
As for airbrushes, there's single or dual action; internal or external mix; and gravity or siphon feed.
Single action means you have one button on the brush that controls the airflow only - the paint flow is fixed (or maybe it's the other way around?). Dual action has a single button on the airbrush that controls both paint and air flow - you adjust either depending on how you move the button. Many beginners prefer single action because the learning curve is shorter and it takes a little more finesse and practice to master a dual action. However, you can get finer detail with dual action, and many people prefer to spend the extra time up front learning how to use them because they are more versatile later on when the user becomes more skilled with the airbrush.
Internal mix means the air and paint mix inside the brush before the mixture is spit out the nozzle. External mix means the mixture occurs outside the airbrush, where the air and paint streams meet. I've heard that for acrylic paints (which is most common in model rr), external mix is preferred because the thickness of acrylic paints tends to gum up the nozzles of internal mix brushes.
Gravity feed means the paint is dispensed into the brush from a little cup mounted on the side of the airbrush. It usually flows down into the brush housing. Siphon feed means the paint is sucked up into the brush from underneath - this is the preferred method for most model rr applications. With siphon feed, the paint is mixed in a little glass jar and attached to the bottom of the brush. You'll want to get an intake screen filter to put over the intake siphon tube to keep little paint flakes, globs, and other debris from gumming up your brush.
The most commonly recommended manufacturers are Badger and Paasche. I don't have their websites handy, but if you Google the names, you can easily find them - I recommend you check out their websites to learn more about airbrushes.
The problem with brush-painting is you are left with uneven paint and brush strokes. The problem with spray cans is generally they tend to put too much paint on where you don't want it - goes on too thick and covers fine details, etc. Airbrushes give you just the right amount of paint where you need it, with a smooth finish. You'll find it's a valuable tool, but you just need a little practice to get the hang of it. I recommend starting with some inexpensive model airplane or car kits from Wal Mart first to master the basics. The two threads linked above are good resources, and you should take a few minutes to look through them to get some more detailed information. Compressors are better than canned air because compressors give you a steady supply of constant-pressure air. Air cans have fluctuating air pressure and are generally not very reliable. Besides, by the time you buy a few cans of compressed air, you could easily afford an air compressor.
I know this is a lot of info thrown at you, but hopefully some of it will be useful.
