actually you should consider what you really want in the lens.
lenses like those 90/100/105mm f/2.8 and sigma 150mm f/2.8 tend to be more popular because they have more usable focal lengths when used outside of macro purposes. also, their f/2.8 is comparatively faster to the longer 180mms' f/3.5 aperture.
BUT, if you're looking for a pure macro lens, the 180mms are the best in the class. yes, they are slower and longer, and require a tripod/monopod and macro flashes to fully optimize the results, but they also give the best macro results.
so while most settle for a more versatile package, i'd recommend you to get one of the 180mms if you're serious into macro photography.