If you get the nice 100L
If you do a lot of macro work, then the 180L would make sense. It offers a longer working distance, which is helpful in a lot of instances. If you do some macro work and would like to use it for portraiture, then a 100mm makes sense. The IS offers you more flexibility when you try to handhold it, but is not necessary. If you are happy with the ef-s 60 on the crop, then the 100 will perform similarly on FF. If you have a 70-200L II, then the 100L makes less sense because both perform similarly for portaits at 100mm.
+1 ... and if you get the 100L buy the Kenko 1.4x tc which works nicely with it and gives you more working distance while keeping it a real lens that can focus to infinity unlike macro extension tubes (though these can be useful, too). Just don't expect the IS on the 100L to make any difference while shooting handheld near macro 1:1 magnification, a common fallacy (and Canon marketing will make sure to imply otherwise...).
1. A TC does not increase working distance.
2. Since you are going to FF, note that The Kenko TC does not work with the 100L and a 5D MK III
, it just locks the camera up and the battery must be removed to get the Camera /TC combination to work again on a compatible lens.
3. I was very skeptical about getting the IS to work on near macro shots, but it surprised me. I put the camera in AI Servo and it does very well for a handheld macro shot. I have had several non-IS macro lenses, they are all sharp, but for just walking around and getting up close to snap a near macro photo, it is suprisingly good.
Here is a handheld shot with the 100L:
And yet another: