I had the Canon 100 as my first macro lens, and bought the Sigma 105 for my wife, so have used both a bit. You won't notice any difference in them optically. Typically the true macro lenses are very sharp. Also the autofocus on them you might find pretty slow, well, slower than your other lenses. This doesn't matter too much because for macro you'll probably be using manual focus anyway.
Difference between the Canon 100 and Sigma 105 is that the Canon stays the same length when focusing. The Sigma barrel extends as you focus closer. Doesn't really bother me that much, because when shooting macro I usually dial the lens in to whatever focus distance I want first, then move forwards or backwards to get the subject in the depth of field. It may bother you if you have a flash mounted on a bracket and you have to adjust the flash head position every time to change magnification/focus. I don't consider this a big deal, but some may.
Another option I'd recommend is the Tamron 90. Friend had it. Again, very good optics, good price too. I believe the aperture blades are curved. For most macro lenses, because the depth of field is so narrow, you usually get really nice bokeh in the macro range. But for normal, terrestrial, shooting I've seen the Tamron 90 give some really nice bokeh, particularly on specular highlights, points of light. Think JJJnettie here has one, and I've seen some of her astro widefields taken with it. The stars seem to have nicer shapes, not as many of those spikes you see on other lens widefields.
Just thought that might be a consideration for someone on an astro-forum.
