i have a 5d and have the 17 - 40 l lens .. to be honest i have heard great things about the 17 -40 from most people but i found it a disappointment .. not sharp enough and it gave a false effect, from what i can gather the best results come from old lenses like contax,zeiss olympus etc with adapters and use as manual and stop down ...
http://photonotes.org/articles/eos-m...nses/#problems ... in my honest opinion the 5d is so good that most optics struggle ... apart from really expensive l prime wide angle lenses. i imagine for your line of work critical sharpness is not as important and the need for auto focus is etc , so maybe get a 17 - 40 ...maybe i just got a dud one.