Originally Posted by Davesrose
Canon does make a EF 180mm f/3.5L : a little less macro, and a little faster. I would do that over spending a lot for "iffy" results on a lens with different mount. At f/4, I would probably even get the EF100mm 2.8 and stick an extension tube on it even. Heck, with the Sigma 150mm 2.8, you have even less light fall off from a smaller extension tube if you wanted to go bigger in magnification still.
Um, the point of getting a longer macro lens isn't for magnification, it's for working distance. At f/4, I would still much rather have the longer lens instead of the extra magnification or aperture speed because it allows me to keep my distance when I'm shooting, whereas using extension tubes or whatever just requires me to get closer. One of my photographic heroes, Tom Hicks of FM, says that when it comes down to macro lenses for shooting insects, especially dragonflies and butterflies that don't let you get as close, he needs all the focal length he can get. If Canon made a 200mm, 250mm, or 300mm 1:1 lens, he would get it. However, since we don't have the luxury of access to such lenses, we end up relying on teleconverters and close-up filters to get the distance we need, which are in some cases a much greater compromise to image quality than a mere mount adapter. And yeah, Canon (as well as Sigma and Tamron) makes a 180/3.5 that is a great piece of glass, but even if the Nikon is only 20mm longer, it's only half a stop slower, and I would gladly trade in half a stop for an extra 20mm. And, like beerguy mentioned, most macro lenses can't keep their specs at 1:1, and as the Nikon probably isn't a full 200mm at maximum magnification, the Canon can't hold f/3.5 at 1:1, either.
But all in all, I'm just speaking out of my ass. I personally traded my Sigma 150 for the Canon 100 because I actually like shooting macro with shorter glass (more compact and easier to handhold) but I'm know from experience that there are shooters out there who are the opposite.