One vote for the 180mm f/3.5. Although I have not used the new 100mm, several friends with it, seeing my images, remark that they wish they had gone for the 180. It's also a beauty of a telephoto.
I also do plenty of handheld macro work, when it is more convenient. The combination of this heavy lens and a pro body is a pretty stable platform.
Here is a link to the Macro category on my blog:
http://bobtowery.typepad.com/bob_towery/macro/Nearly all these were taken with the 180mm. Fair warning, mostly flower images, so if those aren't your thing, skip it.
Good luck with whichever lens you choose!
Bob.