On a 7D 24 isn't wide enough in some scenarios, your effective field of view will be 38mm which isn't wide at all. If you can live with it then go for the 24-105. 24-105 is vastly superior over the 15-85 in terms of build quality and fixed aperture. It also renders details better than 15-85. Distant object seems off on 15-85, do a crop and it becomes that much more obvious.
If you are upgrading to FF in the near future, then do remember that 24-105L is also available as a kit option and you can save a few hundred if purchased with 5Dc.
I hope this helps.
