Well, I own both! There are advantageous to both.
I actually own two 100LG, one with the medium needle and the other with the fine needle.
I also have the 200-20 "Detail" model, which has a graduated scale to dial in the needle position ensuring consistant line thickness.
The 100LG has the benefit of gravity feed, which allows a lower air pressure, which affords more control. However the dual action can sometimes be a bit tricky to keep a consistant line thickness. The dial on the 200-20 locks in the thickness and takes away the guesswork, but the siphon feed requires a higher air pressure which can cause its own problems.
So which brush do I use most of the time? The 100LG! The gravity feed design means less parts to clean up, and the dual action allows me to change from narrow line to wide pattern on the fly. The only time I pull the 200-20 out is to do very fine detail. There is a learning curve to a dual action AB, but it isn't very steep. Heck, there is a learning curve to every AB!
If I had to only have one AB it would be the 100LG.
edit/
Let me also add that some very excellent work can be done with a single action AB. The talent is in the hands and not the tool. As I said I own both kinds, and while I prefer keeping the 100LG hooked up to the hose, I wouldn't get rid of my 200-20 either. It's just that the 100LG is a more versatile airbrush solely due to the dual action. /edit