I would vote for the 28-75mm since your main focus is portraits/people. I have that lens and have taken thousands of pictures with and I absolutely LOVE it. Pefect zoom range for photographing people. Wide enough for group shots, and long enough to get head shots without being right in the subject's face.
I would hold onto your 18-55mm for the odd occasion that you need a wide angle. Maybe you could later add something like the Sigma 10-20mm to augment your 28-75mm at the wide end.
You may also want to check out this thread I started recently regarding my experience when I bought the Tamron 17-50mm:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/124013-tamron-...-k-x-pics.html
I found that, when compared to my other lenses (Tamron 28-75mm f/2.8, Sigma 50-150mm f/2.8, and Pentax 18-55mm), the Tamron 17-50mm was half a stop darker wide open, and a full stop darker at around f/10. So when compared to my other f/2.8 lenses, the 17-50mm was effectively an f/3.3 or f/3.4 lens.
The copy I had was very sharp and produced beautiful pictures, but I really needed f/2.8 for the type of photography I do, so I returned it. A lot of other people have reported exposure irregularities with this lens as well.
I shoot full-manual 99% of the time, but people who use in-camera metering have reported having to add from 1/2 to a full stop of exposure compensation when using the 17-50mm. That would be consistent with my results. I think that for some reason the lens just doesn't open the aperture quite as wide as it should (which could also contribute to the 17-50mm's reputation for exceptional sharpness at f/2.8).