CANON 17-40 VS 16-35
Also check out the Tokina options
Tokina makes two very nice full frame wide zooms now.
When I tested these vs the corresponding Canon lenses you're looking at, I liked both of the Tokina's better and ended up going with the 16-28.
I recommend searching on these and reading some reviews.
Nikon AF-S Nikkor 24-70mm f/2.8G ED
Nikon AF-S Nikkor 70-200mm f/4G ED VR
Nikon AF-S Nikkor 18-35mm f/3.5-4.5G ED