The Canon EF-S 15-85mm is my favourite ‘go-to lens’ for the 7D and the 1 lens option I use when I only wish to travel with 1 lens. Put simply, it has an extremely useful focal range, very good IQ [sharpness, contrast, low CAs good bokeh, etc], build quality (a step above most consumer zoom lenses) and convenient features like FTM USM AF & a 4 stop IS.
The 24-105mm has a less useful focal range on a 7D, very similar (still very good, but not better) IQ, better build quality – and similar FTM USM AF & a 3 stop IS (note: one stop less effective).
For a 7D, I’d definitely stay with the 15-85mm – as the focal length is just so much more appropriate. There is little difference between 85mm & 105mm. 105mm @ f/4 provides a bit extra control of depth of field than the 85mm @ f/5.6. (I have owned a good copy of a 28-135mm, which I know is a bit different again, but it feels more similar to the 24-105mm regarding focal length).
The 15-85mm stands up well to use in a variety of conditions (I’ve taken photos with it in very dusty environments, light drizzle, fog and humid conditions… it did not suffer issues. I would probably have some concerns using it in heavier rain or in the tropics for extended periods of time, though). The 24-105mm is better weather sealed, but even with it, I would not take it into the harshest environments on a 7D. With a 1D body, I’d venture most habitable places on this planet
In addition to my 15-85mm, as zooms I also have a Sigma 8-16mm (love this UWA lens, a great wider complement to the 15-85mm!) and the 70-300mm L (a great high quality portable tele complement to the 15-85mm). When I want fast glass, I like primes (I don’t consider f/2.8 really ‘fast’, so that’s why I don’t have the Canon EF-S 17-55mm f/2.
. Enjoy taking photos with your 15-85mm!