I own the 16-35/2.8L II, and while it's considerably more expensive than the 17-40/4L, it is a fantastic lens, and one I use almost exclusively for 'scapes, and almost exclusively at 16mm.

As Dave pointed out, it has an annoying 82mm filter size, and it was the first EF lens Canon introduced with that filter size.

If budget is a concern, the 17-40 is a very capable lens too, and if you don't need f/2.8 or the extra 1mm of focal length (16mm is noticeably wider than 17mm even though it doesn't sound like much on paper), then it's a very good buy.

I cannot offer any advice about third-party lenses, as I've not used them; but I have had experience with both of Canon's ultra-wide L zooms, and they are both excellent.