The 17-55mm is definitely a winner in terms of image quality, matching that of L lenses, but has build quality similar to the 24-85mm you have. The 24-70mm is what I use on the 7D for indoor sports (usually with flash, since even f/2.8 doesn't cut it sometimes for sports). I bought the 24-70mm and sold my 17-55mm for three reasons: 1) I bought a 5DII, so I wanted a lens that could be used on both; 2) The extra 15mm on the long end helps a lot with indoor sports; and 3) the weather sealing to avoid dust and those times when I end up shooting in less than optimal conditions. For event photography, I'm guessing the 17-55mm's IS wouldn't help you much. However for those times when you are photographing a stationary subject, the IS on the 17-55mm makes it incredibly hand-holdable. I used to take sharp photos with that lens with shutter speeds as slow as 1/4 sec.


Seeing as you've been using a 24-85mm lens already, have you felt that it wasn't wide enough for what you wanted to shoot? As Daniel said, the 24-70mm isn't very wide on the 7D, so take that into consideration as well.


I had issues with dust getting in my 17-55mm, so much so that I actually figured out how to take the front element off and blow the dust out. I've never had any dust issues with any of my weather-sealed L lenses, and I shoot often with them outdoors. That being said, the dust never really negatively affected any of my photos, I'm just too OCD about my equipment I guess...