I went through an extensive excersise when looking for a macro lense. Time after time the Canon 100m macro popped out as the most superior lense. The 180mm Macro did not receive great reviews for an L lense see,
As you will notice on ODP I think the majority of Canon shooters have the 100mm macro in their bag, for good reason. Very fast AF, nice smooth manual focus, excellent build quality! Fair price!