Originally posted by Ash Marc's got a point.
I have both (the 16-50 and 28-75) and default to the 28-75 for weddings, leaving the 16-50 at home. I find that I usually can get far enough away from the action to take a 'wide angle' shot at 28mm but can rarely get close enough for the tight head shots at 75mm. And the 28-75 is rated amongst the best fast lenses in its range - perhaps not as great as the 16-50, but just more appropriate for weddings. (the 75mm perspective is also more flattering for portraits than a 50mm perspective).
I agree with Ash. I have 16-50 and love it to death. I also had Tamron 28-75 (sold) and a year ago used it to shoot a wedding (as a volunteer) of our son. There were two pros hired for the job. One used Nikon and the other one Canon. I was shadowing these two girls shooting alongside with them with my K20D. My results were at par with theirs and several of my shots made it to the wedding album. Tamron is a great lens. I also took it with me to China and Hong Kong and brought back a lot of good shots.