I have the 17-55 and a 24-105 (I know you're asking about the 24-70, but I think this still applies). I have found the extra reach over the 17-55 a big deal for family shots, especially kids. Granted the 24-70 isn't 105mm, but for family shots 24mm is wide enough for me, otherwise for kid shots I'm always at the long end of the 24-105. With the 17-55 I frequently wanted to switch lenses to a 70-200, but then you can't quite zoom out enough on a crop camera- I always found myself wanting to be in the middle of the two lenses. If you are shooting moving kids, the IS won't help you at all. You can probably handhold 55mm at 1/60s or so anyways, and my kids don't stay still for even that long Being able to shoot at 1/15s doesn't add much for moving subjects.


If you have any desire to get the 10-22mm later (or a 5D!), I'd go with the 24-70... Helps with going full frame later (or your brother's camera) and resale. That is my advice for family shooting. If you do a lot of landscapes, or especially interiors, you'll find the 24mm limiting.


As anasidemy 17-55mm is in for repair- the aperture and IS stopped working after about a year and a half. I did get some dust, but definitely can't tell in the pictures. I will say that the 17-55mm is the sharpest zoom I've owned (versus 24-105 and 70-200) in terms of getting consistently sharp images out of it, even wide open. I use a 40D.


I thought the 17-55mm would be a perfect one stop lens - but since getting the 24-105 I never pull out the 17-55mm for family shots (especially now that it is with Canon!) anymore. Could be a different story on the 7D, though - you'll have more pixels to crop with...


A third option is a used 28-70 plus the 10-22mm, if you can find a good one...