What about the Canon 60mm f/2.8?
This wonderful lens deserves the macro name every bit as much as the 100mm f/2.8 or 180mm f/3.5.
One downfall:
A) You have to get closer to your subject than the 100mm requires. Reef tanks are made of glass which tends to block solid matter such as a camera lenses. 4 inches away from a coral with this lens = 12 inches away with the 100mm
The 60mm f/2.8 is an EF-S lens. This really isn't a downfall at all - more like something you should at least be aware of. EF-S lenses will work perfectly with 1.6x crop factor cameras, such as your 40D, but will not cope with with 1.3x crop or 1.0x (full frame) cameras.
This lens will let you take wider shots which may be better for portrait work. As others have suggest 100mm on 1.6x crop camera may be too much if you have a tight working space. As a true macro lens, you can take the 60mm up to 4 inches away from your subject making zooming with your feet for head shots a non issue. That said I generally prefer longer lenses for portrait work and, if shooting a single subject, may favor my 70-200 over the 24-70. The focal length of your lens will change perspective. What you want the perspective to be is up to you as an artist. Sometimes I may prefer a wide angle lens for portraits because of this perspective change.
Shorter length macro lenses also have different bokeh quality compared to their longer brethren. The background won't appear be as out of focus, but you still won't have any trouble isolating your subject. Like the perspective, this could be a positive or negative trait depending on your personal tastes.
Fun fact: At 1:1, or any other equivalent distance, the depth of field is actually the same no matter what focal length you select. The 60mm only seems to have more than the 100mm because you are looking at a larger area.
