I went from the older 28-75 to the newer 17-50 VC - color, contrast, sharpness are quite similar.
I had the Tamron 28-75 for Pentax a few years ago. For the price it's a great lens, but I always get the impression that Tamron lenses are rather poorly built (I also had their 70-200 f/2.8). Both lenses were new and within two years had aperture blades stuck (which was repaired under warranty) and tiny dust particles under the front element were visibile (though not in photos). I've never experienced anything like it with Pentax Limited or Canon lenses.