Hey,

I own the Sigma 150mm and I am very happy with it. I never used the Canon 180mm so I can't compare. Now, since you asked about it, I will assume that the 30mm difference between the two is not important to you.

I only once used the Canon 100mm that you own with my camera in a photo store that I went to. I have to say that this lens is lightening-fast! probably the fastest lens that I used (even though it was for just few mins).
So, don't expect that from the Sigma - sometimes the sigma's AF "hunts" a lot. That's I believe the only downside of the sigma 150mm - besides that, it's an awesome lens and I really recommend it if the primary use for it is macro. In macro you'll probably be manually focusing anyways so you shouldn't worry for the AF performance. I find myself manually focusing with it when shooting macro 99.9% of the time.

Regarding the image quality - it's simply great all the way from f/2.8 and beyond that. I personally didn't compare its f/2.8 to f/4.0 but I've read that f/4.0 is just slightly better than f/2.8 which is already great (just like DavidEccleston above me said).

I suggest that you read this article by "Juza" (an Italian nature photographer) http://www.juzaphoto.com/article.php?l=en&article=63