The work different.
Springs drop by being shorter, and usually are stiffer. Springs are also a fixed height. Air bags replace the springs and drop by the physical release or filling (to raise) w/ air.
As airbags can lower or raise, they are great for the lowest possible drop. You can literally sit the frame on the ground, the raise it back up to drive away. Try that w/ springs and your moving no where fast.
The flipside . . .
Air bags can provide a decently soft ride, but only "handle" so-so in fast corning.
In cornering (like on mountain twisties), you'd really want springs for optimum handling. Also, when looking at springs, there are some designed for handling first, while others are designed for maximum drop first. As such, the design compromises of each make for a different "look" vs. "handling" compromise.
As I am interested in handling primarily and only drop second (I could be "OK" w/ little to no drop if they were the best handling springs on the market), at the top of my list are the Hotchkis springs. Hotchkis designs their units for handling, but don't drop as much as the DFs.
As my personal interest is in handling, airbags won't be seen on my fridge.
That said, I've seen WONDERFUL airbags xBs, mainly first gens. In Scikotics, Boxzilla, Spazzy Spaz (Tim at SparksTRD), and "Crabs O'Nasty" (Chris, of -I think- Jacksonville's Panda Auto Works
http://getpaw.com/index.html ) all have really cool airbag units. THOSE made me actually seriously consider a set!
In short, it gets to what YOU are interested in and can afford. What is important? Absolute drop? Comfortable ride? Extreme/optimized handling? The ability to raise and lower?
Springs: Good drop, variable ride (based on brand and design- how stiff), usually improve handling (some more dramatically than others), FIXED height (or adjustable w/ a decent amount of work).
Bags: Extreme drop, good to great ride, OK handling (wouldn't autocross it), wide range of ride heights at the push of a button/toggle.
