16-35 may be worth the difference if you're going to be needing f2.8 but otherwise the 17-40 is a great wide angle lens (on full frame body) and perfectly fine for landscapes. I also use mine on a 1.3 crop 1D3 if out in the pouring rain.
I'm the opposite in that I'm probably going to add the 24-105 to my 17-40 as I've currently a gaping hole between 50mm and 100mm just now.
I can't comment on that particular Sigma.
Phil,
I have just taken delivery of the Sigma 24-105mm (Canon fit). I have not used it in anger yet but it is one solid lump of lens and is very well put together. The nice thing is that I can tweak the focus, should I need to as I also bought the Sigma USB dock. As you are considering getting a 24-105 may I suggest you put the Sigma on your list of options.
Andy
I was looking at that Sigma although I'd probably be buying second hand so may struggle.
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.
I have a MK lll with a 24-105 lens and I am interested in landscapes and have been looking at the canon 17-40 ( can't afford the 16-35) and the sigma 12-24 mk ll . Would be interested in any opinion from anyone with either