Canon 24-70 f/2.8 L
Sigma 24-70 f/2.8
The Canon does seem crisper, but £600 odd crisper? I've upgraded for a few reasons. The front end on my Sigma has become wobbly causing it miss-focus from time to time. I'll be getting it repaired. I'm increasingly needing faster focusing speed so I wanted USM. I got mine from Kerso so I didn't have to pay Jessops prices. I wouldn't have upgraded if I had to because I don't feel its £600 I could justify on USM and build quality. The Sigma is a great lens for its price. Nice and sharp. I've got great results from it and I don't regret buying it at all.
So if you want a lens in the range of 24-70 that'll do well in low light and you're on a budget, get the Sigma. Its great. If you can afford it go for the Canon because it will be more durable and quicker to focus.