I apologize in advance because my comment isn't about these specific lenses, but rather about Sigma lenses as a whole. I have not had good experience with Sigma brand lenses in respect to good autofocus. This could be due to them having to reverse engineer to get their lenses to work with Canon gear, I'm not sure. I owned a Sigma 18-125 mm for a while and was never happy with it. When it would focus properly it was tack sharp. The problem was it would miss focus more times than not. Shortly after buying it I found it had a front-focus issue as well so I sent it back to Sigma for repair. Sigma put in a new chip and it came back with the front focus problem fixed but unfortunately it still mis-focused a lot. I just couldn't chance missing a good shot due to bad focus so I sold it. I saved my pennies and sold a couple of kidneys on Ebay and got a 17-85 IS and a 70-300 IS and couldn't be happier. If you are on a tight budget and have no alternative than to buy third party then maybe sketchy focus is something you can live with. It could be that I just had a bad copy. Unfortunately I've read very similar opinions on DPReview.com as well so this is definitely something you should think about before buying.
Good luck to you.