17-40 = lighter, slower, filter size is the same as my other lenses.
16-35 II = heavier, faster
I've used both for landscape photography and I like the 17-40 more just because it's lighter and for the price difference, I can buy another prime lens. If you pixel peep, 16-35II is a bit sharper at the edges at open-wide apertures. However, they're almost the same from F8 and smaller. Since I use mainly F8 and smaller (most of the time F11-F13), I didn't have any trouble choosing 17-40. At night, I mostly use F1.8 primes which generally are better than the 16-35 especially when stopped down to the same max aperture of F2.8. If you're using it for events or anything where you need more light from your lens, then by all means, 16-35 II is a must. If it were me though, I prefer the newer versions of Samyang 14mm F2.8 (it's not an L though and isn't weather sealed). It's cheaper, wider and sharper.