I shoot with 1.6x bodies and I own both the 17-55mm f/2.8 IS and the 24-70mm f/2.8L lenses. Both of these are excellent lenses. I have an older copy of the 24-70L. I have noticed that there seems to have been more complaints about this lens in the last several years than when I purchased it. All reports were, at that time, glowing regarding the 24-70L.
As I said, both my 24-70L and my 17-55mm IS lenses are excellent but, I tend to use them in two different ways. However, I seriously doubt that in real-life, day-to-day use; you will notice any great difference in the image quality between these two great lenses in the final resulting image.
I first owned the 24-70mm f/2.8L lens and used this lens in combination with 70-200mm f/4L IS and Tokina 12-24mm f/4 ATX lenses. I would use this combination with at least two 1.6x cameras and sometimes carry a third camera with lens mounted. I absolutely hate to switch lenses during shooting and since I cut my teeth on film cameras with prime lenses, using multiple camera bodies is second nature for me.
The three lens system gave me an unbroken focal range of 12-200mm and excellent IQ and autofocus capability throughout the range. It was, however a fairly heavy kit to carry and, even if dispensed with the third camera. The 24-70L upon which I based this kit is a pretty big and heavy lens. I never considered an extreme range zoom such as 18-200mm since these zooms do not provide the image quality I demand, are slower in auto-focus and will have a considerably slower maximum f/stop.
I wanted to cut down the weight of my system for travel purposes and decided that the 17-55mm f/2.8 IS lens might just about cover both the 24-70L and 12-24mm lenses. Obviously, I would not be as wide as with a 12mm focal length but, I thought that I could make do with 17mm as my wide side since I am not a great fan of UWA shooting. When I shot film, 28mm was the widest that I wanted or even needed.
The 17-55mm f/2.8 IS and 70-200mm f/4L IS lens combination has become my go-to travel and general purpose photo combination.
1. I have a very decent focal range and don’t miss the gap between 55mm and 70mm.
2. I have a constant f/2.8 aperture and IS capability in my mid-range zoom. This provides me with a very good low light capability.
3. The two lens/two camera combination is relatively light weight. At least in comparison to the 3 camera and 3 lens combination based on the 24-70L.
However, I like my 24-70L as my main studio lens because:
1. The extra weight doesn‘t bother me because I am not carrying it around for 8-10 hours.
2. The 24mm wide side is not constricting since my studio is rather large and I zoom with my feet.
3. The lack of IS is no problem since I always shoot with studio flash.
4. I like the 70mm long end because IMO it is better for head and shoulder portraits than 55mm.
5. The image ratio I can achieve with the 24-70L is larger than with the 17-55mm and therefore, I can shoot some close up shots without resorting to a macro lens.
That all said; I bought the 17-55mm f/2.8 IS lens while I owned the 24-70mm f/2.8L and I like using these two lenses for their particular strengths. However, if I had purchased the 17-55mm f/2.8 IS lens first, instead of the other way around, I seriously doubt if I would have bought the 24-70L.