When I bought my Canon 5D Mark III last summer, I used my Canon 17-40 f/4 for interior shots but felt that I needed something a little wider. I often take photos of Church interiors.
I bought the Sigma 12-24 f/4.5-5.6 Type II in August and it was significantly wider! I looked into the Type I also as it was much cheaper (almost half the price). But I decided on the Type II because it was a little sharper in the corners and lighter in weight (although theses differences were not that significant).