My 2 Cents on the matter is that if you are considering going up to F2.8 then get the EF-S 17-55mm F2.8. It gives you a nice range and I am sure everyone will vouch for its outstanding image quality.

I have a 60D and 17-55 f2.8 is my highly used lens. I have a 50mm F1.8 (great value for money but not good as the 17-55 optically). I was eying the Sigma for a while just for the f1.4 but I opted to get a Cheap studio strobe and a softbox (50$ all together) from eBay and a Yongnuo flash ($40) for when i out and about.

I am very happy with the image quality and value for money for my setup.