Had the same decision to make a couple of months ago for my 7D. I went for the 24-105 over the 15-85 as I have paired it with my 10-20 sigma which gives me the wide angle if I need it, which is very rarely nowdays. As a general walkaround lens I find the 24-105 a great lens which give a good focal length range, @ 105mm is especially good for portraits. I don't find that 24mm is too narrow for anything other than landscapes and only then in particular circumstances, and I'm not particulary fond of the the distortion of UW angles on crop sensors anway, prefering 18-20mm shots. I'm very happy with the purchase and don't really miss the wide end, but I have another lens to cover that should the need arise.
From what I have heard the 15-85 is also a great lens, and I'm sure you'll be pleased whatever one you choose.