Hate to make your choices more difficult, but a macro lens isn't usually needed for butterfly pics, and not for most flowers, either.
I nearly made the same comment when I read "flowers", but my experiences with butterflies are different. I have done very nice shots with my 70-300L(x1.6 on crop), but the macro shots from my 100mm(x.1.6) lenses are much better, esp. on smaller animals. When they sit still, you can crawl up to them slowly so the macro working distance esp. with the 180mm should be more than adequate - it just takes more tries than from tele range. So yes, it's not "needed", but no, shooting butterflies is a good reason to buy a macro lens.