Andy, have you got any shots you can post with the Sigma 24-105. I've got the Canon and the IQ on my 5D3 is excellent, the AF speed is great and the IS works really well. I'm not planning on changing to the Sigma but I would like to see some samples from 'real world' scenarios.
OP, I got a Canon 17-40, not just for landscapes though, I'd been to an air museum with a 24-105 and was disappointed that I couldn't get a full photo of some of the planes due to space limitations so I went for something wider to ensure I got the whole subject next time. It worked really well when I went back and I've even started taking a few landscapes with it, even though I'm really not that interested in that type of photography, and I'm really impressed with the lens. It's probably going to be my least used lens in the bag, but it's a great tool to have when it's needed.