Hi,
I had the Sigma 24-70 and I now have the Canon 24-70. The difference is night and day.
Issues with Sigma:
Orange color cast, lacked contrast, noisy AF (VERY!!!) , slow AF, back focus issues, very very eratic focus. I was lucky enough to have this lens stolen from me
My BIGGEST gripe was focus - it was just awful. Never accurate, taking photos was always a hit and miss and my results are very similar to what people on Fred Miranda find.
I replaced it with the Canon. I'm much happier
The Canon is light years ahead. AF is silent, and far faster. The contrast is far better and this lens resolves imo alot more detail. Bokeh is superior also and the colors are far far better. View my Flickr for some 24-70 shots
I'd never buy a Sigma lens again over the L series. I suggest if you can afford it, go the Canon as you won't regret it. The Sigma just had far too many quality issues but I guess for $400 what can you expect.
Best of luck!