i havent looked into the specs on the Sigma but i will say that i'm personally looking at saving 500 bucks by purchasing a used Canon 28-70 2.8L. if you need that 24mm wide end then obviously the sigma or canon 24-70 are your only choices (or the canon 17-55 or the canon 24-105 which i hear are fantastic) BUT if you are alright having 28 as your widest then i havent heard many amazing things about the 28-70.
Might be worth having a look at the Tamron 28-75 2.8
Lots of great reviews on that one.
With your crop body you may want to look at the Tamron 17-50 VC or non VC lens also
The 24-70 SIGMA IMO is junk! I had one and took it back. You could not get a sharp picture out of it to save your life, and yes I did try two of them and they both worked the same. Go with the Canon for sure.
I had a 24-70mm for my old Nikon D700 and was always able to get sharp pictures from it. I tested it out along side the Nikon version and couldn't tell the different.
Remember that glass is an investment. The 24-70mm was my walk around lens and bumping into things was less scary knowing that the lens wasn't too hard to replace because of the price.
