The 17-40 is about half the weight too, I believe. You might consider that the typical price and weight difference is double between F4 and F2.8, so this sounds about right, but the 16-35 was updated and improved recently, the 17-40 was not.
The 17-40 is a fantastic landscape lens, mainly daylight. The 16-35 has a bit more purpose with low light and wide open options, you even get some DOF if you are close enough.