tested the 17-40 for a weekend, soft (= unsharp) + horrible CAs.
Had the 16-35/2,8II for more then a year. Never sharp corners. Even at f8.
Didn't like it. Was usable on the 1d4, but FF: sigh....
Bought my first non- Canon lens. Tokina 16-28/2,8.
Own it for 2 years.
Now I am happy. I have read a lot about varying quality, so it seems this time I had good luck.
Sharp from 2.8 on, corners good, corners excellent from 5.6 up.
Never saw this on the 16-35/II....
Be aware of that. It is heavier too. Has only 16-28mm.
I am also not sharing Neuroanatomists opinion, that a bulb- like front needs more loving care in developing.
I recommend to test a copy before you buy.