I've been grappling with the same issue. Started with the 40D and 70-200 f4L is and the 50 f1.4. Fantastic.
What to do about the shorter range? I also want the short zoom equivalent of the 70-200 f4L is, but of course, they don't make it. Rented the 24-105L is and the 17-85 to see how much worse the 17-85 would be. Surprise, surprise. It was sharper! And of course, lighter and smaller, both good things as far as I'm concerned. I think I had the exteme ends of the bell shaped curve in both lenses, because I tested another 17-85 vs the Tamron 17-50. In this test, the 17-85 had problems compared to the Tamron, which was quite good.
So, it is the Tamron for me. (At least until Canon produces that f4, L wide zoom from about 15-60)