I think you just fail to see that the 100mm is a great lens. It's MTF figures are higher than both of those other Sigma's AND the 180mm f/3.5L. I have yet to find a bug that I can't get close enough too to shoot at 1:1 or so.
Now ..... the MP-E 1x-5x is a whole other story.
I would go with the 150mm, it gets much better reviews than 180mm.
If you want to shoot insects, I can guarantee that you are going to want to shoot in the f/11 range, because of the shallowness of the DOF at 1:1.
The Sigma does give you a longer working distance (about 3 inches longer). But this made me a little nervous when I read it:
Probably just a fluke, but still makes me nervous. I was looking to pick one up in addition to my 100mm, but decided to spend the money on a macro friendly tripod...
I don't know about the 60mm, but the 180L has LOWER resolution figures than the 100mm. The 100 is FREAKISHLY sharp and an excellent lens. I find it has plenty of working distance, even for most jumpy subjects.
Eventually I would reccomend a ring light or an off-camera flash setup. While...
How often do you use your tripod, and what do you use it for?
About 40% of the time I guess. If the subject is being nice and not moving then I will try to get the tripod out so I can focus stack easier.
What do you think is the most important piece of kit to own and why.