my colleague had a 16-35 and i could compare it with my 17-40
he has the benefit of shooting at 16mm vs my 17mm, but his lens distorts buildings much more than mine
and i felt that the difference in price didn't justify the gain of 1mm
so far i'm very happy with my 17-40
Well I would say thats furthest from the truth of difference .
I too owned both and love my 16-35II, and enjoyed the 17-40L as well, but it doesn't even compare to the same shortness towards centre at all. Big difference.
The biggest thing is the sharpness, next being the 2.8 vs 4 depending on if you need the extra light or not, and then the extra 1mm being minimal to non importance.