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Canon seems to offer a better selection of macro lenses than does Nikon. Moreover, Nikon macro lenses can be used on Canon camera, with an adapter, but not vice versa, which also suggests Canon to be the better choice. Third party lenses, however, should be considered, which makes the Canon-versus-Nikon choice largely moot.
I started DSLR photography with a Canon XTi, which is a Canon USA name for what was sold in most of the world as the 400D. My wife bought it from a co-worker, with an small assortment of lenses, the best of which is the Tokina 100mm 2.8 Macro, a quite good lens, which Tokina makes for both Canon and Nikon cameras. The body of the lens does extend during focusing.
I later bought a Canon EF 100mm Macro 2.8L, which is sealed against dust and moisture, and does not extend while focusing, which are sometimes important factors for me when using it at work. (I use this lens extensively for crime scene/forensic photography, sometimes in the rain or heavy fog.)
Canon makes an EF 180mm Macro, which is very well-regarded, which allows one to have a bit more distance from living subjects, such as insects and spiders, which might not be comfortable at the working distance of shorter focal length lenses. Nikon's counterpart is a Nikkor 200mm, which also
seems well-regarded. I do not yet have personal experience with either of these, though both are being considered for future purchase, as I am not totally devoted to either Canon or Nikon cameras, and use both.
The Canon MP-E 65mm 2.8 1-5x is a truly remarkable lens, usable only as a true Macro lens, as it cannot focus at infinity, as can the other Canon macro lenses. A local acquaintance, who is otherwise a Nikon-only shooter, bought a Canon 60D, specifically so he could use this particular lens. I have seen some beautiful images he has taken with this combination.
If one wants to shoot insects and other creatures, keep in mind that the various 40mm to 60mm macro lenses on the market require the shooter to get very, very close to the subject to achieve a 1:1 or 1:2 image, and most living subjects are not going to tolerate that, as they will, understandably, think the lens is about to eat or crush them. Keep in mind that the distance listed in specification charts, unless otherwise stated, is measured from the camera's sensor, not the outer element of the lens.
I am not a highly-experienced macro shooter, so I hope others will reply to this thread, with their
experiences, and to comment on the lenses, especially the Zeiss, Tamron, and Sigma macro lenses.