Numenorean wrote in post #18502239
I don't know what I'm shooting. Whatever I happen to find

Probably flowers and leaves and such, perhaps small mushrooms or any other interesting thing I might come across.
I would prefer at least 1:1.
They both provide 1:1 reproduction ratio. The difference is the working distance.
The 180L will do it at 48cm, or 18.9 inches.
The 100L will do it at 30.5cm, or 12 inches.
Consider that approximate, assuming a full frame sensor. It's different on smaller sensors.
But basically, the 180L lets you do 1:1 from significantly farther away, which allows for more natural settings and light potential on larger subjects that are spooky. Big subjects (like butterflies, flowers, etc) can be done from over 3 feet away with this 180mm lens. And if you did it on an APS-C sensor, or used a 1.4x or 2.0x TC you could get even more distance out of it, or 2:1 magnification out of it. With a 2.0x TC you could get 1:1 basically at nearly three feet. Think about that. This is why you move towards 180mm when you need the working distance while still having the magnification.
I shot 180mm for a long time, even with a 2.0x TC just to have insane 2:1 reproduction at minimum focus distance, or 1:1 at nearly 3 feet. But, I ultimately went back to the shorter lens with image stabilization because I like being able to drag the shutter, hand held, and I found I really didn't need the working distance because my subjects were not spooky. Just my preference.
Very best,