Both theEF-S 17-55mm f/2.8 IS and theEF 24-105mm f/4<span style="color: #ff0000;"]L IS are excellent lenses. IMO, the 17-55mm is the best general purpose zoom lens for a 1.6x body like the 7D. 24mm x 1.6 is 38mm FF-equivalent, meaning at the wide end, the 24-105mm is technically not even a wide angle lens on a crop body. I really don't think you'll miss the 55-70mm range. The only other consideration is weather-sealing - the 24-105mm is a sealed lens (note that a filter is required to complete the seal), meaning you can use your 7D with it in the rain if you want - not so for the 17-55mm. But that aside, I'd really recommend keeping the 17-55mm. As you state, faster aperture, and provides a wide angle.


As Sean states, if you plan to get a FF body very soon, then the 24-105mm is a better choice. Alternatively, if you have an ultrawide zoom like the EF-S 10-22mm f/3.5-4.5, that combined with the 24-105mm and the 70-200mm II would deliver excellent versatility.


Personally, although I have a 5DII and love the 24-105mm on that body, I kept my 17-55mm for use on my 7D.