Hi Dan
When you say butterflies, do you want them sitting still or in flight? Makes a difference which lens you use.
When I used Canon/Nikon, found the Sigma macro to be a better choice, both optically and size/weight. It is the only non-sytem lens ever purchased, they do have one huge disadvantage which is resale value, not easy to sell and don't get much back. Buying used is very attractive.
Focal length wise, the 180 would be the best choice ... more distance to subject. btw the lens mentioned by Jan (great) is actually more expensive than the Canon lens !!!!! Was not aware of that model, has IS. One fine lens, have not used personally that model but checked out some reputable reports and impressive it is. Even with that lens, same resale problem will be encountered.
For catching them flying, a 70-200 would be the best, I have some samples in my album Various others. Those were taking with a Sony A7 II, not sure if the Canon would be able to track them in flight as well?
Whichever lens you decide, do get some extension tubes, go for the 25mm and get at least two ! Looking forward to seeing your butterflies !!
al