This is bad, this summer I could not make the same choice, so I got the tamron 17-50 2.8, because of the cost and its 17mm was to be wider that the canon's 17mm, and have been very happy with it, except for some back-focusing at very close range. You seem very well informed about the choices, have you looked at the new EPS 15-85 ( I think that is the right model ), wider on both ends, IS, somewhere inbetween for cost, and a little slower F-stop wise?