Obviously working distance is not an issue when shooting flowers - I get that.
However, I do really want to get some nice butterfly photos.
Hate to make your choices more difficult, but a macro lens isn't usually needed for butterfly pics, and not for most flowers, either. Usually, a 0.25x or so magnification is sufficient for butterflies - what you really need is working distance, and the 9.5" with the 180L often isn't enough. At 4-5 feet distance, you'll get higher mag with the 300/4L IS (0.24x maximum magnification, great for flowers and butterflies).
With all due respect - I believe the minimum focussing distance on the 180 is about 18 inches - and that is for 1:1 magnification
Yes, but also with all due respect, I didn't state the minimum focus distance is 9.5", I stated that is the minimum working distance. MFD is measured from the sensor, ignoring the physical length of the lens. Working distance is measured from the front element - in my experience, butterflies don't get startled and fly away because the sensor is 18" away, they fly off because the business end of the lens is even closer...