Originally Posted by Canada_Rockies 
The 50mm would be of more use for someone photographing inanimate objects - stamps, jewelry, etc. The depth of field is roughly double at the same magnification and aperture, and the perspective is different as well.
The 100mm is a better choice for taking pictures of creepy crawly things that fly away when you get too close.
The FA* 200mm f/4 Macro would be even better, but my wife won't let me mortgage the house to buy one.
The depth of field at the same magnification is roughly the same for the 50mm and 100mm lens, as you would be at about half the shooting distance with the 50mm lens, compared to the 100mm.
The only real difference is (as Dave illustrated so nicely) is the perspective, which is different at the same magnification, as you would go nearer with the 50mm for that. And perspective only changes with shooting distance.
Other than that, I fully agree, that longer fl macro lenses are very useful for anything biting, stinging or shy. I would love the A 200, but have to stick with my Voigtlander 180mm, instead...
Ben