Neither can substitute one another. I love reading posts where people say they dont miss the 24-70L after buying a 35L. I find this incredibly humorous since the 35L looses versatility. You can look at a folder of 35L images and find them all to have normal perspective. With a zoom you can change FL aswell as perspective.
The current 24-70L is an "ok" zoom. Its age is really showing and its not one of my lenses that I can call stellar. Its a workhorse with weak points aswell as strong. The 24-70Lmk2 is a no brainer for someone wanting canon's newest fresh revisions that seem to be delivering incredible IQ. If you look at the 24Lmk2 and 70-200 f/2.8 IS MK2 those are examples of big jumps in IQ over the older version.
For demanding users I would say unfortunately the wallet will go thin and you will eventually need both lenses to fill your requirements of getting the shots.
