Sigma vs Canon 17-50/55

I'm heavily considering these two lenses for my 7D but the price differences are making it challenging to choose. I'd love to save some money but I am also wanting something to match my 70-200 f4 in AF speed and accuracy.

Is the Sigma as fact auto-focusing as the Canon is said to be and accurate as well? (I've read about Sigma having QC issues with focus with some of their lenses).

I'm trying to decide the best way to spend my money and get a fantastic lens.

You may wish to review the thread over at http://www.dpreview.co​m/forums/post/40076389 which also introduces the Tamron equivalent into the comparison.

When I purchased I did not include the Tamron but did the Canon/Sigma comparison and ended up buying the Sigma. To be honest I really found it difficult to justify the difference in cost. The store let me shoot numerous images with both lenses and I was able to return home to do some pixel peepping prior to purchase. The lens I actually bought was given a battery test while in the store and it tested spot on far as focus and I have no regrets on the purchase. I don't have any low light focusing issues with the lens on my 60D.

Note that at least at present, the Sigma is shown with a $100 instant savings on the Sigma website (http://www.sigmaphoto.​com/lenses/standard). In addition Sigma is finally releasing the lenses in the new "adjustable" series. If you are after a fast lens you may wish to consider the 18-35mm f/1.8.

I own the Sigma and have not had any issues with focus speed or accuracy. I am also not aware of the Sigma zooms having the autofocus issues that particularly seemed to affect some of their prime lenses.

That said, if your goal is autofocus performance that matches a Canon L lens, you should probably stick with the Canon lens.

I have a Tamron 17-50 f/2.8 and have no problems with the AF focusing in normal or low light with a 60D.

To be honest the Tamron's AF is just as quick as the the AF on my Canon EF 200mm f/2.8L mk2 and that's a fast prime L lens!

My Tamron 17-50 f/2.8 works just fine on my 60D. I use it in one-shot and ai-servo and have no complaints at all about the focus speed or accuracy. Used it was 1/4 the price of a the Canon new, and 1/3 the price of the Canon used. No brainer, really.