Unless you need that extra stop of light you are paying double the price for no increase in performance, dont forget weight of the lens will also be different, every kilo hurts in landscape photography As for reviews and scientific tests yada yada yada who cares! Its one persons opinion and i have yet to read a test i havent managed to find issue over the test criteria. The sigma 12-24 is a good lens, not great but ok- it has a projecting front element so filter work is nigh impossible, you said your a landscape shooter so filters are a must im sure.
The Nikon is an astonishing performer, but anyone looking at a wide angle manual AF spending that much money should look at the Zeiss optic, the distagon range is perfect, it makes the canon and nikon lenses look like kit lenses.
Oh and you can actually fit an EFS lens to full frame it requiredsripping the back off the lens to mount it but it does work, you have to watch the projection into the mirror box though, i have seen the 10-22 on a 5d work at 12mm and the results arent that bad, do a search on flickr and you will find an example im sure.
Canon appear to have filed a patent application for a new and distinctly different design for a wide angle lens. You can view the application here
If this is right then we can expect a new wide angle lens to compete against Nikons 14 - 24mm f/2.8 rumours on Fredmiranda are saying anything from 14 - 24 to a 10 - 35mm (which would be very difficult to make) all at the f/2.8 aperture.
One thing is for certain, I won't be buying a wide angle lens in the forseeable future unless this is released soon.