I have the Sigma 30mm, and the AF is noticeably slow. Does this lens have that same problem? I'd be willing to pay more for the Canon 17-55mm 2.8 if the AF works better on that one.
I've used the Sigma 30mm and Sigma 50mm before, and I agree, the AF on those were horrible. They were slow and would go back and forth between the MFD and infinity to search for focus. The focus on the Sigma 70-200 (non-OS) was also not the best.
The 17-50 on the other hand is by far the best focusing Sigma lens I've used. Rarely did I have an issue with it not finding a focus point. Unless you're shooting in the dark, I don't think the AF will be an issue at all.
If you need FTM, definitely go for the 17-55. There's no thinking about it here because the 17-50 does not have FTM. However, I don't think the cons of the Sigma--softer corners at 2.8, slight vignetting at the wider end--is enough to spend $150 more on a used 17-55mm.