Some things you haven't told us that might be important:
  1. What sort of photography will you be doing? The "better" lens for nature photography would be different from sports or portraits, etc. For nature photography, for example, I'd suggest a 100mm f/2.8 Macro ($525) plus the EF-S 17-85mm f/4-5.6 IS ($515, total $1040) about the same as the 17-55mm f/2.8 IS ($1030). You could even throw in the 50mm f/1.8 ($115, total $1155) and third-party hoods and still be less than the 24-70mm ($1270).
  2. What other lenses do you have? For example, if you have the 24-105mm f/4L IS, there may not be much point in the 24-70mm f/2.8L--it's a lot of money for 1 stop plus there's no IS. If you have a 70-200mm f/4L or f/2.8L, then I'd recommend the 17-55mm lens, as it is considerably wider and you wouldn't as much need the longer focal length of the 24-70mm. If you have an 18-55mm f/3.5-5.6 IS, then the 17-55mm won't buy you a lot except the f/2.8 aperture.
  3. How wide do you want to go? On your 50D, the 24-70mm is equivalent to a 38-112mm lens on a full-frame camera. That's not very wide, at all. On the other hand, the 17-55mm is equivalent to 27-88mm, i.e., almost what the 24-80mm would be on a full-frame camera.
  4. Why do you want an f/2.8 lens? Is it for the speed (low light, stopping action), the background blurring or ??? You could buy a series of primes for the same price that would be as fast or faster:

  • 24mm f/2.8 $310
  • 35mm f/2 $300
  • 50mm f/1.8 $115
  • 85mm f/1.8 $380
  • Total = $1105



If you answer those questions, the right lens might become obvious.


Another factor to consider is that, as Nhut said, the 1.6x bodies aren't going away. (They probably outsell the full-frame and 1.3x bodies by several times.) Thus, you can always get rid of the 17-55mm lens if you switch to a 5D. I paid $780 for a used 17-55mm lens, so it, like the "L" lenses, holds its value pretty well. (That was about the average price over the past several months. I've seen the lens sell for as much as $850-890.)


I have the 17-55mm f/2.8 IS and would like to also get the 24-70mm f/2.8L. The reason is simple: I shoot indoor sports (horse events) where the f/2.8 aperture is very helpful. I have the 70-200mm f/2.8L IS lens, but the IS doesn't help a lot because of the shutter speed I need (1/400-1/800) to stop the horses' legs in motion. The 24-70mm would fit just under the 70-200mm lens, but, for wider shots, I'd still need the 17-55mm. Even then, I may have a 35mm f/2, 50mm f/1.8, 85mm f/1.8 and/or 100mm f/2 for really fast action.


When I do nature (wildlife, birds, flowers, etc.) photography, I carry a 100-400mm f/4.5-5.6L IS (plus 1.4x extender, to go to 140-560mm f/6-8) and the 17-85mm f/4-5.6 IS. They're usually fast enough for outdoors shots. I may also have a 100mm f/2.8 Macro along, though a 77mm Canon 500D close-up lens may suffice. Sometimes, I will include a Sigma 10-20mm lens for super close-ups.


For flowers (gardens, etc), I now use the 17-55mm lens mostly because it has better image quality than the 17-85mm that I used to use. (I don't use f/2.8 very often because of the very shallow depth of field.) I usually also have the 100mm Macro & 70-200mm f/4L IS. The f/4L IS is less than half the weight of the f/2.8L IS
lens (1.6 lb vs 3.5 lbs!) and is actually "good" handheld to slower
shutter speeds because of the greatly superior IS. If I don't need the
action-stopping or background-blurring of the f/2.8 lens, why carry
that big, heavy lens around? (I had the 70-200mm f/4L IS lens for quite
a while before biting the bullet and getting the f/2.8L IS lens.)


If I'm shooting people indoors, the 17-55mm usually fits the bill, though, for available light, the 35mm f/2 and 50mm f/1.8 can be quite useful. For "walking around" outdoors during the day, the 17-85mm is lighter and has a longer range than the 17-55mm lens. It may be accompanied by a Sigma 10-20mm and perhaps the 70-200mm f/4L IS.


So, you may see that there's no one answer that fits everyone well.[*]