Considering the well-documented focusing problems Sigma lenses have with Canon bodies, I'd never buy one without having several copies on hand that you can test out. There's also a chance of Sigma lenses not working with future Canon bodies, since they reverse-engineer as opposed to licensing from Canon. Also, Canon 100-400 has IS (2-3 stops, iirc).
On a completely different note, bluebirds were back at my in-laws' backyard again this year. Unfortunately, I didn't have time to grab my glasses or put in my contacts, so I ended up taking the below shot at f/2 when I thought I had stopped down to f/4.'