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...