You have to be careful about what are called 'Wide angle' lenses.
Digital SLRs are based on the 35mm film standard because that was the standard for decades before digital came around. On a 35mm film SLR, a 24mm lens was pretty wide and 16 or 17mm was really wide.
Now, with digital SLR cameras like yours, the image sensor is a little bit smaller than a frame of 35mm film...and the image from the lens is cropped. The result is that when you use a DSLR like yours (as compared to a 35mm film camera)...the view is narrower. This is the 'Crop Factor'. For you camera is 1.6.
So when you see that the 17-40mm lens is listed as a 'Wide Angle'...it will only be a little bit wide, not really wide.
You have the 18-55mm kit lens correct? That should show you how wide 18mm will look on your camera. Either of the two lenses that you listed, will be only the slightest bit wider than your lens at 18mm.
The crop factor, therefore, makes it harder to get a wide angle view on our digital SLR cameras, as compared to film.
So...the camera makers gave us ultra wide angle lenses, designed for the smaller digital sensor.
The Canon lens is the EF-S 10-22mm. I own this lens and it's fantastic. It's a bit expensive though (but not compared to the ones you listed). Some people will recommend the Sigma 10-20mm lens.
As for why those two are so different in price. The 70-40mm has a max aperture of F4 and the other one has a max aperture of F2.8. That is usually enough to add several hundred dollars. The 17-40mm F4L, is also considered a great deal because you get the quality of an 'L' lens at a 'good' price. Most 'L' lenses are closer to the 16-35mm price.
'L' is Canon's line of Pro lenses.