madmacs => If you're particular about composition, some shots from some angles just cannot be helped. Only high ISO or IS to help.
I wouldn't go so far as to say that. It looks like there's significant distortion. I've not seen the 16-35 or 17-35 on FF against this lens, but my gut feeling is that they are be better. That said, I think you should limit your words to 17-40 is
not bad for full frame. I must say 1st though, I've not touched a FF camera with wide angle lenses on. From what I see though, I'm not impressed with the 17-40. I thought many lenses were shot down for their distortion?
To the thread starter - Both lenses are not fantastic lenses as per say, but if you shoot in lower light conditions more often, I'd think the 17-85 is better. You WILL get more keepers , definitely. F4 is just not fast enough.
however if you just shoot outdoors, might as well get the 17-40f4L. You'll get slightly sharper images which would make most photographers who pixel peep happier. Given the choice though, there are quite alot of other better 3rd party lenses out there which cost less, is faster (f2.8) and sharper than the 17-40f4l.
Of course, the 17-40 f4l has got USM to make up for it's slightly smaller maximum aperture and slightly softer images. This swings the boat around for many people.
Not to mention though, that the ability to boast that you have an L lens comes along with it. However that interest is better served if you were to get a white lens instead.

people will SEE, don't need you to tell.. hahaha...