Welcome aboard! I'd go for the Tamron 17-50mm, because the extra stop and better IQ are worthwhile IMO.
Sensor-shift SR is just as good as VR at all but the longest focal lengths, and it stabilizes your primes, which is a huge plus. I can get perfectly-sharp pics at 1/15s with SR. Did a comparison of my K-5 vs a D800 and the results were both just as sharp.
Here are some comparative samples with SR on vs SR off:
Pentax K-5 Camera Review - Image Tests - PentaxForums.com Adam
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