All true macro lenses will focus close enough that you will get 1:1 magnification of the subject.
This means that the image on the sensor will be the same size as the object being photographed. So there's no advantage there for any of the true macros.
But there are various focal lengths available. And that means that some of them are more "telephoto" than others. Thus, you can be farther from your prey with the longer focal length lenses than you can be with the shorter focal lengh ones.
If the subject is easily scared, or you have to "reach into" a bush or something like that, then a more telephoto lens will be helpful. But the tradeoff is that (just as with any telephoto lens) the longer focal length ones are more difficult to hand-hold.
Canon makes a 180, a 100, and the EF-S 60. Tamron and Sigma and perhaps Tokina also make excellent macro lenses as well.
A true macro lens will focus from 1:1 to infinity, so the lens will be useful for things other than macro shooting too. Thus, one question you might want to ask yourself is:
"What focal length of lens might I like to have for non-macro shooting?"
That way, your macro lens can perhaps fill in a gap in your lens lineup.
I have the EF-S 60mm macro lens and I love it. The 60mm focal length on a 1.6X body is just great for portraits. And it's a fantastically sharp macro lens too. It's small, light, easy to handle, and the autofocus (for distances over about 2 to 3 feet) is the fastest of any lens I own. It's even faster to AF than the highly touted 85mm f/1.8. It'd make a great sports lens if the focal length was what you wanted
But mainly, it's just a dandy macro lens.
However, I see posts constantly from people shooting all of the other macro lenses, and I have yet to see any that I don't like. So consider what your needs will be, and pick a focal length that will do you the most good.
Macro shooting is a blast. I think you'll love just about any macro lens you might get.
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Jim H.