There are basically three types of macro lenses: (1) a normal macro such as the 50mm macro; (2) a medium telephoto macro such as the 100mm or 105mm macro lenses; (3) a telephoto macro such as the 180mm or 200mm macro lenses.
Here are the advantages and disadvantages:
1. Telephoto macros have less depth of field. The background goes out of focus more, which is usually desirable, but the subject has less dof, too.
2. The working distance is longer between a telephoto macro lens and the subject. The typical focusing distance is four to five feet. This is perfect for shooting subjects like butterflies where you can't approach them closely. A 50mm macro lens requires you to get much closer -- within inches -- to fill the frame with the subject and focus. A 100mm macro is somewhere in between. Usually you will shoot about two feet away from the subject.
3. A 50mm macro is much cheaper, much smaller, and much lighter. For backpackers or people who travel, this is a big deal.
I use the 50mm macro lens because it is so small and light. If I want a telephoto macro lens, I simply use extension tubes on my 100 - 400mm telephoto. This gives me a telephoto zoom macro!