It has been discussed and debated at length, sure. And it will likely be debated long after all of us here have lost the eyesight to continue shooting too.
I own the Tamron and wouldn't trade it for the DA*16-50. Now personally, I don't like Sigmas. That however is purely a matter of personal taste as optically I think they are fine lenses; they just don't feel right to me. Other people have exactly the opposite reaction comparing Tamron and Sigma.
In general, as far as speed and sharpness goes, the rule of thumb I go by is most modern lenses enter their sweet spots when stopped down 1 to 2 stops from full open. Max aperture has as much to do with depth of field control as it has for low light shooting, but when it comes to lower light situations an extra stop from f/4 to f/2.8 does mean you get two times as much light hitting the film/sensor at the same shutter speed. And in low light, I have more issues with color noise due to higher ISO than I do with any lack of optical sharpness at f/2.8 on the Tamron 17-50.
The real issue for me though when I'm shopping zooms is constant versus variable max aperture. Zooms allow me to frame my shot the way I want it when I either can not or do not desire to move my feet and the perspective I have of the scene from my location suits me. In these situations I'm not counting the millimeters of focal length but I am always concerned with my aperture and shutter speed. And for this reason I don't like variable aperture zooms. I find it very frustrating to zoom in a just tad tighter on a shot (it may only be a 10mm adjustment) and find I've dropped from 1/60s @ f/4 to 1/30 @ f/5.6. It's why I just consider my DA18-250 to be an f/6.7 lens and I put up with it until I find a better option or my little boy requires less gear of his own when we "go to the zoo".
So which one to buy? I'd advise putting your hands on each if possible and going with the one that feels the best to you. Optically they're not going to be very different but how they feel to
you, in your hand, mounted on the camera, can make a world of difference in the photos you make.