This may be a slightly old thread, and the OP may have made his decision, but since I currently own the first two lenses I'll chime in with my personal experience. Disclaimer: I'm only an amature photographer who loves to take pictures.
I have the Tamron 17-50 f2.8 non VC. The image quality of this lens is great. The lens is light and easy to carry. The AF is quick but noisy, and in dim light it will often make 2-3 micro adjustments following the first autofocus movement. I've found that when using a flash the focus is sketchy, and the keeper rate goes down unless you're using only the center focus point. I don't like that the focus ring turns when autofocusing, and there is no full time manual focus. The zoom has just the right amount of dampening to make it smooth, without turning too easily. It has no gravity creep. Wide open you can get a pretty shallow depth of field, but the bokeh is not as smooth as high end zooms like the Canon 24-70. In general the lens is a great step up from a kit lens, and asside from a couple of quirks my wife and I have been quite happy with this lens.
The Canon 17-85 is a good lens, and was a huge step up from the 18-55 kit lens we had initially. On my 40D it works fine, but doesn't have nearly the low light capabilities of the Tamron. Its a good walk around lens, especially outdoors. I've found the autofocus with this lens to be fast, smooth and accurate. The only place that the AF searches is in really low light. Usually it will lock on, even in situations where its too dark to get a reasonable picture. I like that with the USM the AF ring doesn't turn while autofocusing. I have found that the zoom ring has loosened up quite a bit since we got the lens 2 years ago, to the point where it will gravity zoom in certain situations. This is my biggest compliant about my 17-85.
I don't have the Tamron 28-75, and have never used it, but a good friend of mine (who is a lot more serious photographer than me) had one for a while. He was always complaining about the autofocus of that lens. He said that his keeper rate for low light was well below 50%.
Between the two lenses that I have direct experience with, I think I would go with the Tamron. It is more flexible with the faster aperture, and I can overlook the loud AF. I think the image quality is better from the Tamron, and its slightly narrower and lighter to carry. I rarely miss the 50-85mm focal length, and I have a telephoto lens for those times when I want it. I hope this helps.